The aim of this paper is to examine the Suarecian theory of practical reason and its first principle, which is also the first principle of natural law. Although our main source is Suarez's De anima, other texts are used as well, including De legibus ac Deo legislatore and Disputationes metaphysicae, wherein the Jesuit author introduces some relevant insights about the first principle of natural law. This paper attempts to show that Suarecian practical reason operates independently of speculative reason (because both have their own first principles) and that natural law is a norm of reason, i.e., a practical and directive principle of human actions.