free will, in philosophy, the doctrine that an individual, regardless of forces external to him, can and does choose at least some of his actions. The existence of free will is challenged by determinism. A denial of free will was implicit in Plato's argument that, because no one would deliberately choose a worse over a better course of action, people's decisions are determined by their understanding (or ignorance) of what constitutes the good. Aristotle disagreed; he distinguished between reason and desire, pointing out that people sometimes do what they desire even when they know it will harm themselves or others. Some Stoics sought to adapt the idea of free will to their rigorous form of determinism; Chrysippus emphasized that action could be produced by choice which itself had antecedent causes. In the Christian philosophical tradition a central question regarding freedom of the will was this: is virtue within the power of the individual or completely dependent on the power of God? St. Augustine, although he argued that God's foreknowledge of human actions (a consequence of his omniscience) did not cause them, did hold that God's omnipotent providence implied predestination: man was wholly dependent on divine grace. St. Thomas Aquinas maintained the freedom of man's will in spite of divine omnipotence, holding that God's omnipotence meant he could do all things possible or consistent with his goodness and reason, which did not include the predetermination of human will. William of Occam affirmed free will but claimed it impossible for any human to comprehend how it is compatible with God's foreknowledge and omniscience, which cannot be distinguished from his role as prime mover and original cause. Martin Luther and John Calvin both followed Augustine's doctrine of predestination, but later Protestant writers disputed their position. Advocates of free will have usually begun with the overwhelming testimony of common practice and common sense: people do believe they in some way determine their actions, and hold each other accountable for them. Therefore advocates of free will have argued that the human will, unlike inanimate things, can initiate its own activity. This position has been called into question by experiments, first undertaken by American neuroscientist Benjamin Libet in the 1970s, that have shown that brain signals associated with decisions concerning actions occur before a human being is conscious of making a decision.