Christian August Crusius (January 10, 1715 – October 18, 1775) was a German philosopher and theologian. He enjoyed a considerable reputation in Germany during the 1750s and 1760s, standing out in stark opposition to the then-dominant rationalist Leibnizian-Wolffian philosophy (this label, though dating back to Crusius' own time, is somewhat misleading; see the articles on Leibniz and Wolff). Crusius' philosophy is marked by an insistence on the important of the non-intellectual side of human nature, in a way that often echoes the empiricism of Locke and Hume. In his philosophical system, religious and practical considerations are often given a more central place than theoretical and secular considerations, though Crusius shows none of the anti-intellectualism of others in the Pietist movement with which he was associated.
Only recently have Kant scholars begun to fully realize the profound impact that Crusius' thought had on Kant's theoretical and practical philosophy. Sadly, there are no full translations of any of his major works into English.
Christian August Crusius was born at Leuna, near Saxony, on January 10, 1715. His father was a pastor, and Crusius was interested in theology from an early age. Crusius' education, at the University of Leipzig, was deeply influenced by his teacher Adolf Friedrich Hoffmann, a follower of the philosopher and theologian Christian Thomasius. Hoffmann was a staunch opponent to the hyper-rationalist Christian Wolff, and his sympathy for Pietism and his emphasis on the limitations of pure reason and the importance of contingent psychological principles became central aspects of Crusius' work. Crusius stayed at Leipzig as a professor, first as a professor of philosophy and later as a professor theology. His most important works, originally written in German, appeared in the 1740s: Advice for a Rational Life (1744), Sketch of the Necessary Truths of Reason (1745), and The Way to the Certainty and Dependability of Human Knowledge (1747). Crusius also wrote a number of shorter works (many in Latin) into the early 1770s. He died in 1775.
Crusius has often been described (not least of all by Kant) in contrast to Wolff. Though their philosophical systems differed dramatically, the contrast is in fact brought out clearer by certain methodological similarities. Like Wolff, Crusius aimed at systematicity, thoroughness, and clarity. The thematic scope of their philosophies is comparably broad, ranging from the most general metaphysical questions to what today would be described as applied ethics.
Wolff, radicalizing a central part of Leibniz's thought, had claimed that all truth was based on the principle of non-contradiction (that, for any proposition P, it is impossible for it both to be the case that P and for it to be the case that not-P). This meant that the crucial method for arriving at philosophical truth involved the analysis of concepts and the development of proper definitions, which are essential for discovering hidden contradictions. Crusius believed in the importance to philosophy of the principle of non-contradiction, and the use of analysis and definitions, he denied that this all was sufficient as a general philosophical methodology. Many philosophical truths, he believed, relied on other, 'material' principles that could not be reduced to 'formal' facts about logical consistency. In other words, Crusius believed that there were principled philosophical reasons for rejecting certain apparent possibilities even though those apparent possibilities were logically consistent. These material principles had their basis in the nature of the human understanding. The general picture Crusius described is striking in its similarity to Kant's central distinction between analytic a priori truths (that is, truths that can be established independently of experience merely by considering the logical consistency of a proposition or its denial) and synthetic a priori truth (that is, truths that can be established independently of experience, but not by any considerations of the logical consistency of the proposition).
Relatedly, Crusius also developed a point of Adolf Friedrich Hoffmann's that was to reappear both in Kant's 'Prize Essay' of 1764 and in Kant's mature philosophical work. According to some rationalist pictures (the most vivid example of which may be that of Spinoza), philosophy can proceed in exactly the same way as mathematics; that is, by setting out basic definitions and axioms, and then deriving further results. Against this, Crusius argued that the basic concepts with which philosophers begin are 'rich' and in need of preliminary analysis. For instance, a mathematician can simply define "triangle" as a three-sided polygon, and has no reason to worry about the adequacy of that definition. By contrast, a philosopher can offer a definition of, say, "substance" as something that exists independently of other things, but there is a substantive question as to whether the philosopher has got the right definition. A close analysis of the concept may be needed before one can go on to make any claims involving that concept.
A crucial principle for Leibniz and Wolff was the principle of sufficient reason (which Wolff attempted to derive from the principle of non-contradiction). The principle, which played a crucial role in Leibniz's arguments that the actual world is the best possible world and that space and time must be relative, claims that, for every truth, there is a sufficient reason (or explanation) of why it, and not its opposite, is the case. Leibniz was hard-pressed to reconcile such a view with the possibility of free action (human or divine), for the principle appears to rule out situations in which someone is genuinely free to pursue or not pursue a certain course of action. Unimpressed with Leibniz's attempts at compatibilism, Crusius rejected the principle wholesale, for he believed that completely free will was essential for theology.
In a discussion closely resembling a famous argument of David Hume's (of whose work Crusius was probably unaware), Crusius claimed that no logical relation could be found in our judgments of cause and effect. In other words, when we assert that the throwing of a rock caused the breaking of a window, this is not because we think that there would be some contradiction in the throwing of the stone occurring without the window breaking. Rather, Crusius claimed, we simply come to be unable to vividly think of the stone's being thrown without the window breaking, and this limitation in thought derives merely from associations based on experience.
Wolff had claimed that there is no real distinction in souls between their representative capacities and their volitional capacities. For Wolff, when we intellectually represent a possible state of affairs as good, we thereby will that it attain. Crusius insisted that there must be a fundamental distinction between the understanding and the will, so that no intellectual perception was capable of determining the will one way or the other. What we pursue in willing is determined by a set of natural desires, which include a love of truth and a love of God. These natural desires are God-given, and when we act on them properly we are fulfilling God's will and attaining ethical goodness.
Elsewhere in his practical philosophy, Crusius made the distinction between performing some action in a way that merely happens to be is accordance with duty from performing that action because it was a duty. This distinction plays a crucial role in Kant's argument in the first book of his famous Groundwork for the Metaphysics of Morals of 1785.
Throughout his career, Crusius was deeply concerned with the relation between philosophy and religion. On one hand, he saw religion (in particular, revealed religion) as a source of truths that could not be arrived at by any purely philosophical methods (this being one of the clearest examples of Crusius' Pietist sympathies). On the other hand, Crusius believed that a rigorous philosophy was needed both to defend religion from philosophical attack, and to demonstrate the incoherence of certain false religions.
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