The 19th-century philosopher Friedrich Nietzsche is known as a critic of Judeo-Christian morality and religions in general. One of the arguments he raised against the truthfulness of these doctrines is that they are based upon the concept of free will, which, in his opinion, does not exist. Schopenhauer In The Gay Science, Nietzsche praises Arthur Schopenhauer's "immortal doctrines of the intellectuality of intuition, the apriority of the law of causality, (...) and the non-freedom of the will," which have not been assimilated enough by the disciples. Following is, then, the short description of those views of the latter philosopher. The principle of causality In Fourfold Root of the Principle of Sufficient Reason Schopenhauer claimed to prove – in accordance with Kant and against Hume – that causality is present in the perceivable reality as its principle, i.e. it precedes and enables human perception (so called apriority of the principle of causality), and thus it is not just an observation of something likely, statistically frequent, which however does not happen "on principle" (empiricism of the principle of causality). More on this dispute in philosophy can be found in the article on free will. Physical freedom In his treatise On the Freedom of the Will, Schopenhauer calls the fact that we can do whatever we will a physical freedom, i.e. lack of physically present obstacles, which is not identical with moral freedom. Physically "free" means: one acting according only to one's will; if attempts are made to use this term to the will itself, the question arises: "is will itself willed?," "do you will the will to become so-and-so?". It is therefore a specific aspect of the claim of freedom, in which it is stressed whether the course of consciousness follows indeed in a willed way. The problem of willing the will appears in Thus spake Zarathustra, for instance in the chapter "Backworldsmen." Necessity vs. contingency In On the Freedom of the Will, Schopenhauer demonstrates the (well known in philosophy) distinction between necessity and contingency. He calls "necessary" what follows from a given sufficient basis (i.e. that what is already certain – if one knows that the sufficient cause is present). On the other hand, one calls "contingent" or "incidental" (with regard to a sufficient basis) that what does not follow from the latter (so e.g. two unconnected events can be contingent to each other: like when a black cat crosses the