  Like many representatives of 19th-century logic and philosophy of science, Jevons starts his investigation by examining the nature of the Laws of Thought. Jevons argues that these Laws are true both “in the nature of thought and things”.  Given that science is in the mind and not in the things, the Laws of Thought seem to be purely subjective and only verified in the observation of the external world. However, Jevons argues that it is impossible to prove the fundamental laws of logic by reasoning, since they are already presupposed by the notion of a proof.  Hence, the Laws of Thought must be presupposed by science as “the prior conditions of all thought and all knowledge”.  Furthermore, our thoughts cannot be used as a criterion of truth, since we all know that mistakes are possible and omnipresent. Hence, we need to presuppose objective Laws of Thought in order to discriminate between correct and incorrect reasoning. It follows that Jevons regards the Laws of Thought as objective laws.