Crusius, Christian August (1715?–1775)

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Christian August Crusius, the German Pietist philosopher and theologian, was born at Leuna, Saxony. Educated in Leipzig, he was appointed extraordinary professor of philosophy there in 1744, and professor of theology in 1750. Crusius initiated the third wave of Pietist attacks on Wolffianism by a series of dissertations (17391745), and continued it in his four main philosophical works (17441749). He later turned to theological studies, lost interest in philosophy, and founded a new theological school, the Biblicoprophetic school, which partially diverged from Pietism. He later became canonicus at the Meissen Theological Seminary.

Crusius's reputation in his own time and his influence on his contemporaries was second among Pietist philosophers only to Christian Thomasius. The collaboration of his close follower, A. F. Reinhard, with Pierre-Louis Moreau de Maupertuis and the Berlin Academy in their polemics against Wolffianism established a link between Christian Wolff's Pietist and academic opponents. Several later philosophers acknowledged Crusius as their teacher, although they combined a Crusian background with more advanced trends of French and English origin. These thinkers contributed considerably to the renewal of German philosophy after the dissolution of the Wolffian school. In theology Crusius's influence was even stronger.

Crusius's importance was forgotten or suppressed soon after his death, especially among theologians, and has not yet been fully reestablished because of the hos-tility of the subsequently dominant rationalist and philological schools to the trend of his theology. As a philosopher, Crusius was nearly voted into oblivion, along with most other minor eighteenth-century philosophers, by idealistic historiographers. He was rediscovered by the new philological historiographers, chiefly in connection with his influence on Immanuel Kant.

Origin of Crusius's Thought

After 1730, Wolff and his school began to recover from his expulsion from Halle University in 1723, and from the loss by most of his pupils of their professorships, an attack launched for personal and political reasons by his Pietist opponents. The Pietists were gradually deprived of official support and were more and more restricted to theoretical controversy with Wolff. However, Wolff's system of philosophy was a much more modern, comprehensive, and technically refined body of doctrines than those in the obsolete and clumsy treatises of Thomasius, Franz Budde, and Andreas Rüdiger. A far-reaching reform in the doctrine and quality of Pietist philosophy was needed for it to face the Wolffian doctrine and counteract it successfully. Crusius's teacher, A. F. Hoffman (17031741), developed the logical doctrines of Thomasius and Rüdiger, taking into account Wolff's new philosophical techniques and achievements and accepting some of his doctrines, in his own Vernunft-Lehre (Leipzig, 1737). Crusius's own logic was inspired by Hoffman's refined and comprehensive handbook, whose quality and thoroughness substantially met the most modern requirements. Hoffman's early death prevented him from publishing the treatises on the other branches of philosophy that he had announced in 1734, but Crusius proceeded along Hoffman's lines, both improving and completing his lifework. Crusius provided the Pietist school with a renewed, efficient, and modern theoretical platform that temporarily assured its philosophical survival, outlived orthodox Wolffianism, and led to a far-reaching change in German philosophy.

Methodology and Logic

Crusius's methodology, the foundation of his philosophical attitude, was based on two central ideas, both originating in the Pietist tradition. Philosophy is not, as it was for Wolff, a pure "science of possible things insofar as they are possible," but is based on existing things. Second, human understanding has very narrow limits; theoretical certainty is impossible concerning many fundamental points whose only foundation is moral certitude or revelation. The mysteries of religion are not only beyond human reason, as Wolff claimed, but also contradict it. Something may be unthinkable for human reason that is not so for God or in itself.

Crusius held that the most general principle of human knowledge is neither the principle of identity nor the principle of contradiction, but a principle concerning what we can and cannot think: What cannot be thought at all is false, and what cannot be thought of as false is true. Our notions of identity and contradiction are based on this principle, which he called the principle of cogitabilitas. It is an inner criterion, depending on the nature of the human understanding.

Crusius further held that human reason cannot reach ultimate truth. Knowledge begins with experience, both inner and outer, and in many cases is stopped in its analysis of an order of facts by certain notions that, although they are not simple in themselves, cannot be further analyzed by man. Even if an analysis is completed and man does reach some simple basic notion, this notion cannot be demonstrated or deduced from a unique source. Each notion must be intuited singly by connecting it with concrete examples.

It is therefore impossible, according to Crusius, to assume that the method of philosophy is identical with the method of mathematics. Mathematics deals with very simple properties of things and its objects are exhaustively defined, whereas many notions relating to objects of philosophical thought can neither be known with intuitive distinctness nor analyzed by man. Again, mathematics proceeds only by demonstration and solely on the basis of the principle of identity. Philosophy, on the other hand, frequently must revert to moral certainty and is based on several different principles and on the knowledge of fact.

The main characteristics of Crusian logic, as expounded in his Weg zur Gewissheit und Zuverlässigkeit der menschlichen Erkenntnis (Way to certainty and reliability of human knowledge; Leipzig, 1747), follow from these views. Crusius connected logic with methodology. His logic contained much empirical psychology and many informal concrete and practical rules for obtaining or verifying knowledge, including rules for experimentation. Because Crusius so limited the field of theoretical demonstration, he presented a highly developed logic of probability (which he called moral certitude), covering, among other topics, induction, hypothesis, and the reliability of testimony. The last was essential in the justification of revelation.

Both for Crusius and for Wolff, knowledge derived only from the senses, but the main characteristics of Crusius's methodology allowed his successors to be much more receptive to English and French empiricism, sensationalism, and commonsense philosophy than were orthodox Wolffian rationalists. This receptivity was partially due to John Locke's strong influence on Thomasius, but the ethical and mystical sources of these Pietistic attitudes was most important.


Crusius, in his Entwurf der nothwendigen Vernunftwahrheiten (Sketch of necessary rational truths; Leipzig, 1745), divided metaphysics into ontology, theology, cosmology, and pneumatology, in explicit opposition to Wolff's ordering of the metaphysical sciences.


Ontology begins, not with first principles, but with the notion of a thing in general, directly connected with the notion of a "really given thing." Only after introducing these notions did Crusius discuss essence, existence, and causality. Crusius regarded existence as indefinable and as a primary notion arising from sensation.

In his discussion of causality, Crusius expounded a principle of determining reason, his version of Gottfried Wilhelm Leibniz's principle of sufficient reason. Crusius held, against Wolff, that a sufficient reason suffices only for free actions insofar as they are free. Rational truths and natural events not depending on free causes need a more cogent foundation, a determining reason. This principle does not derive from the principle of identity, but rather from what we must conceive or what we cannot conceive as united or separate, and thus from a new case of the principle of cogitabilitas. Crusius, aiming at a sharper distinction between mechanism and free actions, held that the real nature of causality is unknown and that our knowledge of causal connections is based on the constant conjunction of two events in experience. This, of course, cleared the path for the members of his school to accept the Humean critique of the causal connection.

Crusius's ontology reveals a general characteristic of his metaphysics. His was not a monolithic system beginning with a single principle and deducing from it all subsequent notions and propositions, as was Wolff's. Rather, it was founded both on several independent principles and on a multitude of elementary notions that could be defined only by an appeal to reality (by their concrete representation)notions such as existence, space, time, and force; or, in psychology, the particular powers of the soul, some mental faculties, and pleasure and pain. Through Hoffman Crusius derived this view from Locke's doctrine of simple ideas, but he supposed that the number of elementary notions (which he once called categories) could be infinite.


Rational theology followed immediately after ontology, instead of beingas for Wolffthe final section of metaphysics, because Crusius held that God's existence is a necessary foundation for cosmology and pneumatology. Crusius denied the Ontological Argument: God's existence can be proved by moral evidence only, and his attributes cannot, properly speaking, be understood by humankindamong other things, positive infinity is beyond human reason. The human notion of God is partially relative and partially negative; nevertheless, it is certain. God is different from created beings both in degree and in essence. Among the attributes of God, Crusius stressed his free will, which is limited by the principle of contradiction and by his goodness. In God and God alone, intellect and will are a single power.


Crusius held that matter is composed of a multitude of simple substances. Simple material substances are extended, and the infinite divisibility of matter is impossible. Simple substances have an essential, though not absolutely necessary, force. They act upon each other only by motion and contact. Physical space and time are real, but they are neither independent beings (substances), nor properties, nor relations (all of these concern the metaphysical essence of things). Space and time are intimately connected with existence; they are conditions of things insofar as such things exist. There is no real space or time without substance to fill it; outside the real world there is only possible (not sensible) space or time, which is infinite and filled by God. There are empty spaces in the world (otherwise movement would be impossible), but they are only physicallyand not metaphysicallyempty, because they are filled with God's presence. Mathematical space and time are distinct from physical space and time, and are abstracted from the relations of things.

Crusius was trying to offer a new set of solutions to the difficulties of the traditional doctrines of substance, of space and time and their limits, and of the void, while avoiding the concepts of res extensa, Leibnizian monads, and atoms, as well as the contradictions presented by the real space and time of René Descartes and Isaac Newton and the ideal space and time of Leibniz and Wolff. His doctrine resembled that of Locke, but it was a mixture of well-chosen elements of the traditional views connected by doubtful subtleties.


In his pneumatology, or rational psychology, Crusius rejected Thomasius's spiritual materialism but retained some of its characteristics. He held that finite spirits are simple unextended substances, but that they fill a space and share with material substances the power of motion. Thus, a real interaction between spiritual and material substances is possible, and the doctrines of preestablished harmony and occasionalism are unnecessary. The human soul is an independent substance with two fundamental powers, thinking and willing, both of which are a complex of several independent lesser powers.

Crusius was, in general, very cautious in his pneumatology, and frequently appealed to the limitations of human reasoning. For instance, he held that the immortality of the soul could be proved only if God's existence were presupposedthat is, by an appeal to moral certitude.

Natural Philosophy

Crusius's treatise on natural philosophy, Anleitung, über natürliche Begebenheiten ordentlich und vorsichtig nachzudenken (Introduction to regular and prudent reflections on natural events; 2 vols., Leipzig, 1749), was by far the least original of his works. Nevertheless, he was the first important Pietist philosopher to accept mechanism. In this work, Pietist philosophy finally renounced animism and adopted the more modern Cartesian and Leibnizian views, although it was still opposed to Newton's theory of gravitation. Crusius stressed the difficulties of physics and the purely hypothetical character of much of our knowledge of the particular laws of nature.


Crusius's first major work was a treatise on ethics, Anweisung, vernunftig zu leben (Instructions for a Reasonable Life; Leipzig, 1744). Hoffman's influence on Crusius is clear. Ethics, for Crusius, is not based on reason alone, but also on revelation. Natural duties have been imposed on humanity through God's free choice.

the will

Crusius split Wolff's empirical psychology into two parts. He incorporated the first part, concerned with the cognitive power, into logic. The second, concerned with the will, he placed in ethics. Moral goodness consists in the conformity of the human will with God's will. The human will is a power to act on the understanding, on the body, and on the will itself, but its connection with the understanding is not altogether clear. We are immediately conscious of freedom, which is the main property of the human will. The will is moved by sufficient reason, which does not necessitate, and therefore the will is free.


The second section of the Anweisung, on ethics proper, discusses human duties. An action is moral if it is done out of obligation only, and not in quest of happiness. Virtue is formally conditioned by a coincidence of human will and divine law, and is materially conditioned by love for God. Divine law is known through conscience, which is an immediate power of moral judgment founded on a sort of common sense called moral taste. Evil originates in a wrong use of free will, which, when it submits to unreasonable impulses, corrupts human understanding and the true representation of goodness.

A third section of the Anweisung was devoted to moral theology; a fourth, to natural law; and a fifth, to prudence, which was closely studied in Thomasius's school and partially corresponded to Kant's technical imperatives.

Revealed Theology

In his revealed theology Crusius united orthodox Pietist doctrines with those of a dissident Pietist, J. A. Bengel (16871752). Bengel and Crusius carried to an extreme the Pietist belief that the Bible is an organic whole inspired by God and historically true throughout. The Pietists held that Scripture is the only source of theological truth, and rejected all exegetical developments, even those of Protestant divines. No rational criticism of the Bible was permitted; its meaning could be penetrated only by a kind of empathy or inner light. Crusius stressed a theology of history, founded on biblical prophecies, that tried to explain the whole history of Christianity and to reveal its future aim in a second coming of Christ.

Crusius's Influence on Kant

Recent historical scholarship has stressed Crusius's importance in Kant's development, and the view that Kant's philosophy was rooted in Wolff's system has been more and more questioned. Recent research has shown that Kant, educated in the Pietistic, eclectic, and anti-Wolffian milieu of Königsberg University, was mainly trying in his precritical development (17451768)despite the nonorthodox Wolffian influence of his teacher, Martin Knutzento counteract Wolffian philosophy in an increasingly original way. He therefore appealed both to recent anti-Wolffian trendsto Maupertuis and his Berlin circle and through Maupertuis to Newtonand to Crusius, the new leader of Pietist philosophy and only nine years his senior, whose reputation grew tremendously from 1744 on.

Crusius's influence on Kant consists in six main points, some of which were also held by other Pietist philosophers or by Maupertuis. Crusius stressed the limits of human understanding, a theme that recurs in Kant's writings under different forms from 1755 on. He rejected the Ontological Argument, as did Kant after 1755, and he later rejected all theoretical proofs of God's existence. He assumed a multiplicity of independent first principles; Kant did so after 1755. He denied the importance of formal logic, and simplified it. He rejected the possibility of defining existence, and accepted a multiplicity of simple notions. He rejected the mathematical method as applied to philosophy. Kant adopted these last three positions in 1762.

Kant's Crusianism reached its climax in his Untersuchung über die Deutlichkeit der Grundsätze der naturlichen Theologie und der Moral (Investigations concerning the Distinctness of the Fundamental Principles of Natural Theology and Morals; Berlin, 1764), written in 1762. By 1763 Kant's enthusiasm for Crusius's philosophy was waning, but he did not reject the six tenets above and was still influenced by Crusius on individual points as late as the 1770s. J. Bohatec has claimed that Crusius's doctrines in revealed theology exerted some influence on Kant's late works in religion.

See also Descartes, René; Kant, Immanuel; Knutzen, Martin; Leibniz, Gottfried Wilhelm; Locke, John; Maupertuis, Pierre-Louis Moreau de; Newton, Isaac; Ontological Argument for the Existence of God; Pietism; Probability and Chance; Revelation; Rüdiger, Andreas; Sensa; Thomasius, Christian; Wolff, Christian.


additional works by crusius

Die philosophischen Hauptschriften, edited by Giorgio Tonelli. Hildesheim, 1964.


De Corruptelis Intellectus a Voluntate Pendentibus. Leipzig, 1740.

De Appetitibus Insitis Voluntatis Humanae. Leipzig, 1742.

De Usu et Limitibus Principii Rationis Determinantis, Vulgo Sufficientis. Leipzig, 1743.

Opuscula Philosophico-Theologica. Leipzig, 1751. Reprint of above three dissertations.

Theological Work

Hypomnemata ad Theologiam Propheticam. Leipzig, 17641778.

works on crusius

Adickes, Erich. Kantstudien. Kiel and Leipzig, 1895.

Bohatec, J. Die Religionsphilosophie Kants. Hamburg, 1938.

Burkhardt, Hans. "Modalities in Language, Thought and Reality in Leibniz, Descartes and Crusius." Synthese 75 (1988): 183215.

Campo, Mariano. La genesi del criticismo kantiano. Varese, 1953.

Delitzsh, Fr. Die biblisch-prophetische Theologie, ihre Fort-bildung durch Christian August Crusius, und ihre neueste Entwicklung seit der Christologie Hengstenbergs. Leipzig, 1845.

Festner, C. Christian August Crusius als Metaphysiker. Halle, 1892.

Heimsoeth, Heinz. "Metaphysik und Kritik bei Chr. Aug. Crusius" (1926). Reprinted in his Studien zur Philosophie Immanuel Kants. Cologne, 1956.

Marquardt, A. Kant und Crusius. Kiel, 1885.

Schmalenbach, H. Leibniz, 553560. Munich, 1921.

Schmucker, J. Die Ursprünge der Ethik Kants. Meisenheim an der Glan, 1951.

Seitz, A. von. Die Willensfreiheit in der Philosophie des Christian August Crusius. Würzburg, 1899.

Tonelli, Giorgio. Elementi in Kant precritico. Turin, 1959. Vol. I.

Tonelli, Giorgio. "Kant, dall'estetica metafisica all'estetica psicoempirica." Memorie della Academia delle scienze di Torino, Series 3, Vol. 3, Part 2 (1955).

Tonelli, Giorgio. "La question des bornes de l'entendement humain au XVIIIe siècle." Revue de metaphysique et de morale (1959): 396427.

Tonelli, Giorgio. "Der Streit über mathematische Methode in der Philosophie in der ersten Hälfte des XVIII Jahrhunderts." Archiv für Philosophie 9 (1959).

Watkins, Eric. "The Development of Physical Influx in Early Eighteenth Century Germany: Gottsched, Knutzen, and Crusius." Review of Metaphysics 49(2) (1995): 295339.

Watkins, Eric. "Forces and Causes in Kant's Early Pre-Critical Writings." Studies in History and Philosophy of Science 34a(1) (2003): 527.

Wundt, Max. Die deutsche Schulphilosophie im Zeitalter der Aufklärung, 254264. Tübingen, 1945.

Wundt, Max. Kant als Metaphysiker, 6081. Stuttgart: Enke, 1924.

Giorgio Tonelli (1967)

Bibliography updated by Tamra Frei (2005)

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