The article is an attempt to answer the question of whether a 'free act' is a natural kind term. The answer to this question is of theoretical interest. An affirmative answer to the question leads to the conclusion: a free act can be investigated only empirically. The first part of the article reviews various approaches to the understanding of the concepts of natural kind terms, namely: essentialism, unifica-tionism and promiscuous realism. The second part presents the main views on what a 'free act' is according to different positions in the contemporary analytic philosophy of mind: compatibilism (free will is compatible with determinism), libertarianism (there is free will and the world is not determined) and hard incompatibilism (free will is compatible with neither determinism nor indeterminism). It turns out that in hard incompatibilism, free acts could not form a natural kind. However, there is such an opportunity in compatibilism and libertarianism. For example, we can use the arguments by Antony Flew