Propositional Attitudes: Issues in the Philosophy of Mind and Psychology
PROPOSITIONAL ATTITUDES: ISSUES IN THE PHILOSOPHY OF MIND AND PSYCHOLOGY
This entry aims to characterize the philosophical issues surrounding the propositional attitudes. Particular attention is paid to the arguments philosophers have brought to bear when discussing the existence and nature of the attitudes.
Subject Matter and Philosophical Methodology
Discussions of the nature of mind typically distinguish between two fundamental kinds of mental states or properties. One kind of mental state or property involves states that are qualitative in nature: Examples include raw feels, sensations, tickles, and pains. The other kind of mental state or property involves states that are contentful in nature, "pointing to" or "representing" things beyond themselves: Examples include thoughts, desires, fears, and intentions. This distinction is not unproblematic, since it is not clear whether these two categories exhaust the domain of the mental, nor is it clear whether they are mutually exclusive. However, most philosophers of mind accept that there is some important distinction in this region. Propositional attitudes are often cited as the paradigmatic example of the latter kind of mental state.
As their name indicates, the propositional attitudes are attitudes —cognitive relations such as belief, desire, fear, hope—that a subject bears to what are typically (though not uncontroversially) taken to be propositions. The attitudinal component of a propositional attitude is a matter of how a particular proposition is being taken: Thus Sally can believe that it will rain, hope that it will rain, fear that it will rain, and so forth. In each case the proposition that is the content of her attitude, that it will rain, is the same; what differs is how this proposition is being taken by Sally (believed in, hoped for, or feared). Of course, one and the same attitude can be taken towards different propositions: Thus Sam can believe that the Yankees are a great baseball team, believe that Atlanta is hot and muggy, believe that the office of the U.S. presidency has been demeaned, and so forth. In each case Sam's attitude is the same (belief), what differs is the propositions he believes.
A good deal of the attention philosophers have given to the propositional attitudes is devoted to analyzing the sentences used to ascribe the attitudes. Examples of such attitude-ascribing sentences include "Jones believes that it's raining," "Smith worries that State University's soccer team will lose," and "McSorley wants State University's soccer team to lose." Indeed, the very idea that propositional attitudes are cognitive relations that subjects bear to propositions (a variant of which is defended in Jerry Fodor's 1978 article, "Propositional Attitudes") is advanced on the basis of the surface grammar of the sentences used to ascribe the attitudes. Thus "Jones believes that it's raining" is naturally read as saying, of Jones (the reference of "Jones"), that he bears the belief-relation (= the reference of "believes") to the proposition that it's raining (= the reference of "that it's raining"). Even sentences such as "McSorley wants State University's soccer team to lose," which do not appear to refer to a proposition at all, can be translated (admittedly with some awkwardness) into equivalent sentences that do, or at least appear to, make such a reference: "McSorley wants it to be the case that State University's soccer team loses." Admittedly, though, such a propositionalist formulation may not be possible in all cases of attitude-ascribing sentences. Consider "Williams fears bats" or "Simon loves ice cream."
Whatever their ultimate nature (more on which below), the propositional attitudes themselves have been thought to be extremely important for the study of human behavior. This is seen when we consider how we go about explaining our own and others' behavior in those cases in which the behavior is taken to be intentional (falling in the domain of human action). In such cases, we explain the behavior as the effect of the subject's propositional attitudes. Thus it seems natural to explain why McSorley walked to the refrigerator in terms of her desire for cold water and her belief that cold water is to be found there; or to explain why Jackson ran away by citing his belief that a dangerous lion was coming his way and his desire not to get attacked. Explanations of this belief-desire sort are used by ordinary folk as we go about trying to predict and explain the actions of our fellows in everyday circumstances.
One philosophical question that arises in this connection concerns the status of such explanations. Suppose, as many philosophers do, that these explanations are sometimes true. What sort of explanation do they offer? Perhaps everyone can agree that they are rationalizing explanations, depicting the action in question as rational in light of the subject's corpus of beliefs and desires. But some philosophers hold that, in addition to rationalizing the behavior in question, they also provide a causal explanation of it (see Davidson 1963). If so, then the sort of psychology that appeals to the ordinary "folk" explanations of action—what has been termed folk psychology —can take its place beside other sciences that seek to characterize the world's causal nexus.
It is noteworthy that the causal-explanatory perspective provides an alternative approach to the nature of the attitudes, one that differs from the approach involving the analysis of attitude-ascribing sentences. Where the sententialist approach (as we might call it) assumes that we can understand the attitudes by making sense of our talk about them, the causal-explanatory approach begins by assuming that, whatever their ultimate nature, the propositional attitudes are the causal springs of human action. Taking the latter approach leads one to conceive of the attitudes as whatever plays the relevant causal role in the production of action. Of course, the two approaches might well be complementary: What one learns about the attitudes from analyzing attitude-ascribing sentences might be compatible with (and supplement) what one learns about the attitudes by thinking about them as the causal basis of action. (Indeed, the desire to secure the compatibility of the sententialist approach and the causal-explanatory approach appears to be a core motivation behind Fodor's 1975 hypothesis in Language of Thought, according to which propositional attitudes are tokenings of language-like mental symbols in the brain.) But it is also possible that the sententialist and causal-explanatory approaches will turn out to be in tension, with each one yielding some conclusions not sanctioned by, or perhaps even in conflict with, the other. Settling such a matter is perhaps the main burden of philosophical reflection on the nature of the propositional attitudes.
The Nature of the Contents of the Attitudes
Common to both the sententialist and the causal-explanatory approaches to the propositional attitudes is the idea that the attitudes are contentful mental states. As mental states they are about things, typically objects and properties from the nonmental environment. Take Sanchez's belief that his grandmother smothers him with kisses. This belief is about his grandmother, him, and the property of smothering with kisses. This aspect of the propositional attitudes—their being about worldly states of affairs—raises a number of interesting and related philosophical issues. How does something (such as a mental state like Sanchez's belief) come to be about another thing (such as Sanchez's grandmother) in the first place? What determines what a mental state is about? How does the "aboutness" of mental states relate to other forms of "aboutness"? And finally, what can be said about cases in which a mental state is "about" something that does not really exist—unicorns, for example?
Philosophers have introduced the term "intentionality" to designate the domain of aboutness itself. In speaking of mental states as about the world, we are speaking of their intentional properties, just as in speaking of, for example, the sentence "Morty Morris has a big red wart on his nose" as about Morty Morris's big red nose wart, we are speaking of the sentence's intentional properties. Such properties are also called semantic properties: Both mental states and sentences—and arguably pictures, maps, models, and perhaps other things as well—have such properties. When something, such as a mental state or a sentence, has intentional or semantic properties, and so is about something, we can speak of what the state is about as the content of that state. Talk of the content of a mental state is to be understood in terms of what the mental state represents as the case. So Sanchez's belief has a content, which is what that belief represents to be the case: namely, that his grandmother smothers him with kisses.
It is noteworthy that a belief can represent something that is not the case. Suppose that Sanchez's grandmother does not, in fact, smother him with kisses (it's all "in his head," so to speak). Then, supposing there is an inventory of all of the facts that make up our world, we would not find in this inventory any fact to the effect that Sanchez's grandmother smothers him with kisses. In short, there is no fact that is represented by his belief. But then what is this shadowy thing we are calling the content of his belief, that which his belief represents to be the case? Above we called this content a "proposition," and we can now see the attraction of the view that the content of an attitude is a proposition. For although it is hard to say exactly what propositions are, we can say at least this much: The existence of a given proposition does not depend on the existence of the corresponding fact that would make the proposition true. That is, there can be false propositions. Given that Sanchez's grandmother is not as Sanchez's belief depicts her, the proposition that is the content of Sanchez's belief is itself a false proposition.
The postulation of the proposition as the content of the attitudes raises a bundle of related metaphysical questions. What is the nature of propositions? (Is it essentially a linguistic entity? an abstract one? a mental one?) Do propositions have parts, and if so, what is the nature of those parts? Here we focus on a question bearing more directly on the philosophy of mind: How do propositional attitudes come to have the propositional content they have? More concretely, what makes Sanchez's belief a belief about his grandmother, and not, say, about ice cream sundaes or pink elephants or any of an infinite number of other things? Let us address this by asking which facts fix the content of his belief: Which facts are such that, if you fix them, then, no matter what else is going on in the universe, you have fixed the content of his belief that his grandmother smothers him with kisses? A natural first guess would be that the facts in question are facts regarding the mental image(s) in Sanchez's mind at the time that he calls this belief to mind. On such a view, once we fix the mental image(s) "in" his mind, we have fixed what his belief is about.
But this cannot be quite right. First, mental images do not appear to have the right sort of specificity to fix the content of the propositional attitudes. To see this, imagine a scenario in which Sanchez's grandmother has an identical twin, from whom the grandmother herself is indistinguishable, but whom Sanchez has never met or otherwise heard of. Then the image in Sanchez's mind "fits" his grandmother's twin as much as it "fits" his grandmother. But it seems implausible to think that his belief is about the twin, for he has never met or heard of her. Second, in addition to not having the right sort of specificity, mental images are too unstable and subjective to fix the contents of one's attitudes. This is clearest in cases in which the subject matter of the attitude is an abstract one. Precisely what image goes before your mind when you call forth your belief that 1+1=2? And what image is before your mind when you believe that space is (or is not) infinite? Will it be true that any two people who believe e.g. that 1+1=2 will have the same type of image before their minds? Presumably not. But then how does the image fix the content of their belief? It seems that what they have in common, in virtue of which they both count as believing that 1+1=2, is something other than a particular type of image.
And the same point can be made even in cases in which the subject matter of the attitude is not abstract. Take Roger's belief that Morty Morris has a big red wart on his nose. Since Morty Morris is Roger's best friend, Roger has a vivid mental image of Morty (wart and all). But Mathilde, who (having been told by Roger) also believes that Morty Morris has a big red wart on his nose, has never met Morty, and therefore has no such image. Again it seems that what Roger and Mathilde have in common, in virtue of which they both count as believing that Morty Morris has a big red wart on his nose, is something other than a particular type of mental image in mind.
These arguments (and the examples on which they are based) raise a host of issues regarding how propositional attitudes come to have the propositional content they have. Consider first the relation between such contents and the environment in which one lives and interacts. One plausible account of why Sanchez's belief is about his grandmother, rather than her identical twin, is that his belief was caused and sustained by activities involving one woman and not the other. So it can seem that interaction with one's environment is relevant to the determination of the contents of one's attitudes. Next consider the relationship between language and the content of the attitudes. Recalling that mental images are too unstable and subjective to fix the contents of attitudes, we might ask: Precisely what do Roger and Mathilde have in common, in virtue of which they both count as believing that Morty Morris has a big red wart on his nose? At least part (but only part!) of the answer is that they are both disposed to accept and assert a sentence that means that Morty Morris has a big red wart on his nose. Perhaps, then, among the facts that fix the content of one's attitudes we must include facts regarding the meanings of one's words.
These conclusions highlight one of the bigger controversies in the theory of content. In particular, we have seen at least three types of fact that might be regarded as relevant to fixing the content of one's attitudes. We started off with the suggestion that facts regarding the subject's mental images fix the content of her attitudes, but we moved quickly to include facts regarding causal history and then on to facts regarding the meanings of one's words. These correspond roughly to three distinct theoretical options available with respect to the sort of facts needed to fix the content of one's attitudes.
Content internalism is the view that the only facts needed to fix the content of a subject's attitudes are facts that do not presuppose the existence of anything beyond the subject herself. The view with which we started, according to which the facts regarding the subject's mental images fix the contents of her attitudes, is one version of content internalism. But the content internalist can allow other sorts of facts, so long as these do not presuppose the existence of anything beyond the subject herself; and the most plausible versions of content internalism (for which see Searle 1983) include facts about the individual's use of language, where the meanings of her words are not thought to depend on the existence of anything beyond the subject herself. Of the various arguments for content internalism, one of the most influential is what we might call the argument from "intentional inexistence." Consider, to begin, that one can form a belief which is "about" something that does not exist—as with Ponce de Leon's belief that the Fountain of Youth is in Florida, or Roger's belief that the largest natural number is even. What is more, it would seem possible (though of course highly unlikely) that none of our beliefs succeed in being about any existing thing: Perhaps you are suffering an eternal and systematic hallucination in a world containing nothing but your own mind! But in that case, although your beliefs remain the same (or so it might seem), there are no worldly objects for them to be "about." This suggests that the "aboutness" properties of beliefs should be understood in such a way as not to presuppose the existence of anything beyond the thinking subject.
Many philosophers, unconvinced by this sort of argument, have thought that the internalist view is too restrictive in the set of facts it regards as relevant to fixing the content of the attitudes. A second view, content individualism, expands the set of content-fixing facts to include not just the facts allowed by the content internalist, but also any facts regarding the thinker's own causal history. (See Davidson 1984 and 2001 for an example of a view that combines content externalism, which is the denial of content internalism, with content individualism.) Although the cost of moving from internalism to individualism is that of having to rebut the argument from intentional inexistence—something that forces the individualist to come up with an account of beliefs "about" non-existent "objects"—the payoff of making this move can be made clear in connection with the following development of Sanchez's case. Sanchez has an identical twin, Twin-Sanchez, separated from Sanchez from birth. Twin-Sanchez has interacted only with twin-granny, the identical twin of Sanchez's grandmother. Further, the course of experience Sanchez has with his grandmother is internally indistinguishable from the course of experience Twin-Sanchez has with twin-granny. So, for example, at the very moment Sanchez sees his grandmother wearing a lovely purple vest and making waving motions as she smiles, Twin-Sanchez sees twin-granny wearing an indistinguishable lovely purple vest making waving motions as she smiles; at the very moment Sanchez hears his grandmother singing a lovely melody, Twin-Sanchez hears twin-granny singing an indistinguishable lovely melody; and so forth through time. At one point each of the Sanchez twins, admiring the grandmother in his presence, forms a belief he would express with, "She has a wonderful voice." The natural view is that the contents of their beliefs differ: Sanchez's belief represents his grandmother (not twin-granny) as having a wonderful voice, whereas twin-Sanchez's belief represents twin-granny (not granny) as having a wonderful voice.
The content individualist can easily accommodate this natural view, as the difference in content can be fixed by the facts regarding each twins' causal history (with distinct grannies). The content internalist, by contrast, will have trouble accepting the natural view: Since the twins' course of experiences are internalistically indistinguishable, there will be some pressure on the content internalist to treat the twins as having beliefs with the very same content. (See Searle 1983 for an attempt by an internalist to avoid this conclusion.)
But if the content individualist has this virtue over the internalist, some philosophers have felt that individualism does not go far enough. A third position, which we might designate as content anti-individualism, is still more liberal in the range of facts it regards as relevant to fixing the content of a subject's attitudes. As its name suggests, content anti-individualism is the denial of content individualism. But it is helpful to see why a theorist might deny that "individualistic" facts suffice to fix the content of a subject's attitudes. The controversy has to do with the role of language in fixing the content of the attitudes. In one sense, it is uncontroversial that the meaning of one's words determines the contents of the attitudes one expresses with those words. The controversial matter regards what determines the meaning of one's words. The individualist maintains that no facts beyond those regarding the individual speaker herself—the conditions under which she uses her words, how she herself explicates their meanings—are needed to fix the meaning of her words; whereas the anti-individualist maintains that these "individualistic" facts do not suffice to fix the meanings of her words. The insight (or alleged insight) behind anti-individualism is that individual language users typically defer to, and take themselves to be answerable to, public standards of correct usage. Such standards are not typically fixed by the individual's own word usage or meaning-explications, but instead are fixed by the usage of other speakers (Kripke 1972) and the meaning-explicating practices of the relevant experts in her linguistic community (Putnam 1975 and Burge 1979).
Interestingly, the sententialist and causal-explanatory approaches to the attitudes bear on the debate regarding the nature of mental content. For example, among the reasons offered in defense of anti-individualism, Burge notes in "Individualism and the Mental" (1979) that variations in public standards for the correct use of a word lead to differences in the belief-attributing sentences that would be used to report a subject's beliefs. And among the reasons offered in defense of content internalism are considerations pertaining to the internal basis of mental causes (for which see Fodor 1980). Although neither argument is decisive, each suggests the core motivations for and potential liabilities of the various positions on mental content.
The Metaphysics of the Attitudes: Versions of Materialism
The question regarding the nature of mental content cannot be addressed in isolation from what we might call the metaphysics of the attitudes. What is the nature of the states and properties dubbed "the propositional attitudes"? How do such states and properties relate to the thinker's bodily states and properties? These questions, of course, force us to confront a particular version of the notorious mind-body problem.
The positions that can be taken on the relation between a subject's propositional attitudes and her bodily states and properties correspond to positions familiar from the general mind-body problem. Attitude dualism holds that propositional attitudes are immaterial states or properties of thinking subjects. But as with dualism generally, attitude dualism runs into trouble in connection with the causal role that the attitudes are thought to play: How do immaterial states or properties affect a subject's body? Most contemporary philosophers take some version of this problem to be decisive against dualism. And of these most go on to endorse materialism, according to which all of the objects and properties of our world are material in nature. So we will restrict our discussion accordingly.
Among materialist views we can begin with the view known as philosophical behaviorism, according to which the so-called propositional attitudes really are nothing other than complex behavioral dispositions. Philosophical behaviorism itself (unlike psychological behaviorism) was originally motivated by the verification theory of meaning, according to which the meaning of a sentence consists in the conditions whose obtaining would verify the sentence (establish its truth). Since sentences such as "John believes that it's raining" are typically regarded as true or false in virtue of observable behavior (e.g., John's uttering "It's raining!," carrying an umbrella with him, putting on galoshes, and so forth), the result of applying the verification theory of meaning to attitude-ascribing sentences is that each such sentence is to be regarded as equivalent in meaning to a "behavioral translation," a much longer sentence describing all of the behaviors and behavioral dispositions whose presence would verify the original sentence (see e.g. Ryle 1949). However, this view faces two obvious and devastating difficulties.
First, as noted in Putnam "Brains and Behavior" (1965), the view is either false or unacceptably circular. It is false if the translation of the target sentence ("John believes that it's raining") fails to capture all of the conditions whose presence would be taken as evidence for the truth of that sentence. But in order to avoid falsity on this score, the translation will need to make reference to other attitudes the subject has: For example, John's uttering "It's raining!" counts for the truth of "John believes that it is raining" only if he is speaking sincerely and believes that "It's raining" means that it's raining; John's taking an umbrella with him (or putting galoshes on) counts for the truth of "John believes that it's raining" only if he desires not to get wet and believes that the umbrella (galoshes) will prevent him from getting wet; and so forth. In fact, it would appear that the connection between attitudes and behavior invariably involves other attitudes in this way. But in that case, any attempt to translate a target attitude-ascribing sentence will yield a translation which itself contains mention of other attitudes. On pain of circularity, these latter attitude-ascribing components in the translation must also be translated. But then the problem begins again, and the whole approach appears doomed to an unacceptable sort of circularity.
Nor is this philosophical behaviorism's only problem. A second objection is that philosophical behaviorism surrenders the idea of propositional attitudes as the causes of behavior. Consider: that a sugar cube dissolves in water is the basis for regarding it as water-soluble; so it would be an empty explanation to regard its solubility in water as the cause of its dissolving on a particular occasion. (Compare the doctor spoofed in Molière's play La malade imaginaire : He explained the sleep-producing character of a particular drug to its having a "dormative virtue.") Similarly, if beliefs and desires are dispositions to act, then it would be an empty explanation indeed to regard beliefs and desires as the causes of action.
Given the failure of philosophical behaviorism, the desire to preserve the causal profile of the propositional attitudes within a materialist framework provided the main motivation behind identity theory. Recognizing the role of the attitudes as the causes of intelligent behavior, early identity theorists used the fact (or what they regarded as the fact) that the causes of intelligent behavior are to be found in the states and processes of the central nervous system, to conclude that the propositional attitudes are identical to those states and processes of the central nervous system. (For an early formulation of identity theory, albeit in connection with sensory rather than contentful states, see Smart 1962.) The proposed identity was between property-types : An "attitudinal" property-type (such as the property of believing that it is raining ) was held to be identical to a property-type instantiated by the central nervous system (such as the property of having such-and-such a pattern of neural activation in this-or-that region of the brain ). But this gave rise to an objection from the so-called multiple-realizability of mental states (Putnam 1967): On the assumption that creatures whose underlying neurophysiology is very different from our own might nevertheless be regarded as being the subjects of attitudes, such type-identity claims were much too strong.
Such an objection to type-identity theory acquired additional force in light of the development of sophisticated forms of artificial intelligence. Alan Turing's famous "Turing Test" (1950) taught that a system was to be regarded as "intelligent" so long as it behaved in a way that would lead those with whom it interacted to regard it as intelligent. The implicit idea was that any system with the right sort of functional complexity —as seen in its capacity to acquire and process information from its environment and to use this information to guide its subsequent actions—was to be regarded as intelligent (and hence, given some plausible assumptions, as a subject of the propositional attitudes). The result was what is perhaps the most widely accepted view of the metaphysics of the attitudes: functionalism. According to the functionalist, propositional attitudes are best characterized by their functional or causal profile. So just as it would be a mistake to identify the property of being a carburetor with the property of being made of metal and shaped in such-and-such a way —surely being a carburetor is more a matter of function rather than material—so too it would be a mistake to identify the property of believing that it is raining with some particular property of the body. Rather, a subject has this property, and so counts as believing that it is raining, when the subject is in a state with a certain functional or causal profile—one that is caused in certain characteristic sorts of ways (e.g., seeing rain) and interacts with other functionally defined states to bring about certain effects (e.g., producing utterances of "It's raining!," movements to retrieve the umbrella when leaving, and so forth).
According to the functionalist, the first task in connection with the metaphysics of the attitudes is to specify the functional role corresponding to each distinct attitude. Once that task is completed, the functionalist philosopher can then pass on to empirical investigation the task of identifying what particular physical property realizes that functional role in a given system. (Compare: Once the functional role of a carburetor has been specified, we can go on and ask which feature of a particular car realizes that role.) Such a view is often advanced as part of a "computational" theory of the attitudes, according to which the functional role of particular mental states is best understood in information-processing terms. So formulated, functionalism, as shown by A. Newell (1980) and David Marr (1982), has been popular not only in the philosophy of mind but also in traditional cognitive science.
Of course, having specified what we take to be the functional role of a particular attitude-type (say, the belief that it is raining), there is no guarantee that there will be any state or property of the body or the central nervous system playing that role. Perhaps the very idea that there is a state playing that role is itself part of a mistaken theory of the mind, one whose fundamental postulates (beliefs, desires, and so forth) are as misguided as was the postulation of witches and other spiritual entities by misguided theorists of earlier ages. A number of philosophers, such as P. Churchland (1981), have begun to express such misgivings, arguing that the account of mind which postulates propositional attitudes is part of a "worm-eaten myth" that will be replaced as brain science progresses. Such a view, known as eliminative materialism, is perhaps the starkest version of materialism there is, as it combines a general commitment to materialism with the view that there is nothing in the material world that answers to what we take the propositional attitudes to be. Though clearly radical, such a view has challenged mainstream theorists to further clarify what is at issue in the debate over the propositional attitudes.
Burge, Tyler. "Individualism and the Mental." In Studies in Metaphysics, edited by Peter A. French, Theodore E. Uehling, Jr., and Howard K. Wettstein. 73–121. Midwest Studies, vol. 4. Minneapolis: University of Minnesota Press, 1979. Reprinted The Nature of Mind, edited by David M. Rosenthal. Oxford: Oxford University Press, 1991.
Churchland, P. "Eliminative Materialism and the Propositional Attitudes." The Journal of Philosophy 78 (2) (1981): 67–90.
Davidson, Donald. "Actions, Reasons, and Causes."  Reprinted in Essays on Actions and Events. Oxford: Oxford University Press, 1980.
Davidson, Donald. Inquiries into Truth and Interpretation. Oxford: Oxford University Press, 1984.
Davidson, Donald. Subjective, Intersubjective, Objective. Oxford: Oxford University Press, 2001.
Fodor, Jerry. The Language of Thought. New York: Thomas Y. Crowell, 1975.
Fodor, Jerry. "Methodological Solipsism considered as a Research Strategy in Cognitive Psychology." The Behavioral and Brain Sciences 3 (1) (1980): 63–72. Reprinted in The Nature of Mind, edited by David M. Rosenthal. Oxford: Oxford University Press, 1991.
Fodor, Jerry. "Propositional Attitudes." The Monist 61 (4) (1978): 501–23. Reprinted in The Nature of Mind, edited by David M. Rosenthal. Oxford: Oxford University Press, 1991.
Kripke, Saul A. Naming and Necessity. Cambridge, MA: Harvard University Press, 1980.
Marr, David. Vision: A Computational Investigation into the Human Representation and Processing of Visual Information. San Francisco: W. H. Freeman, 1982.
Newell, A. "Physical Symbol Systems." Cognitive Science 4 (2) (1980): 135–183.
Putnam, Hilary. "Brains and Behavior." In Analytical Philosophy, Second Series, edited by R. J. Butler. Oxford: Basil Blackwell and Mott, 1965.
Putnam, Hilary. "The Meaning of 'Meaning'." In his Philosophical Papers. Vol. 2: Mind, Language, and Reality. Cambridge, U.K.: Cambridge University Press, 1975.
Putnam, Hilary. "The Nature of Mental States."  Reprinted The Nature of Mind, edited by David M. Rosenthal. Oxford: Oxford University Press, 1991.
Ryle, Gilbert. The Concept of Mind. Chicago: University of Chicago Press, 1949.
Searle, John R. Intentionality: An Essay in the Philosophy of Mind. Cambridge, U.K.: Cambridge University Press, 1983.
Smart, J. J. C. "Sensations and Brain Processes."  Reprinted in The Nature of Mind, edited by David M. Rosenthal. Oxford: Oxford University Press, 1991.
Turing, Alan. "Computing Machinery and Intelligence." Mind 59 (1950): 433–460.
Sanford Goldberg (2005)
"Propositional Attitudes: Issues in the Philosophy of Mind and Psychology." Encyclopedia of Philosophy. . Encyclopedia.com. 17 Oct. 2018 <http://www.encyclopedia.com>.
"Propositional Attitudes: Issues in the Philosophy of Mind and Psychology." Encyclopedia of Philosophy. . Encyclopedia.com. (October 17, 2018). http://www.encyclopedia.com/humanities/encyclopedias-almanacs-transcripts-and-maps/propositional-attitudes-issues-philosophy-mind-and-psychology
"Propositional Attitudes: Issues in the Philosophy of Mind and Psychology." Encyclopedia of Philosophy. . Retrieved October 17, 2018 from Encyclopedia.com: http://www.encyclopedia.com/humanities/encyclopedias-almanacs-transcripts-and-maps/propositional-attitudes-issues-philosophy-mind-and-psychology