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Believing that experience and observation provide us with all knowledge, Mill rejects all forms of a priori knowledge: the doctrine that we can have knowledge that is independent of experience [[CiteRef::Macleod (2016)]]. Indeed, this departure is a response to Whewell, and more broadly to Kant, who believed that a priori knowledge are necessary pre-conditions of the mind that enable experience [[CiteRef::Losee (1983)]]. Not only does he reject that knowledge of extension, substance and place as a priori, Mill instead argues that this type of seemingly a priori knowledge is “put together out of ideas of sensation” [[CiteRef::Macleod (2016)]][[CiteRef::Mill (1979)]]. In essence, all of our knowledge, including knowledge that is traditionally thought of as a priori, originates from and is dependent on experience.

Most of Mill’s work against the history of science providing source of and justification for science was in response to Whewell’s “History of Inductive Sciences.” Mill disagreed in principle that the history of science can provide us with a justification for an evaluative criteria for scientific theories. For him, all history of science could provide us is the information that certain regularities have held in the past. Mill made the descriptive claim that scientific inquiry is a search for causal connections---causal relations that are invariable and unconditional. He maintained that all history of science can provide evidence for is that certain correlations have been invariable. However, because he lived in a post-Humean context, he not only inherited the problem of induction, but he also held that induction is fallible. Consequently, he argued that just because certain scientific theories have thus far not been refuted (i.e., they have so far been invariable), it does not follow that they will continue to be invariable. As induction is fallible and because scientific theories are nothing more than ‘refined induction,’ the theories themselves are fallible---there is no guarantee that scientific theories will remain invariable in the future as well [[CiteRef::Losee (1983)]]. As a result, he thought it nonsensical to study the history of science to find the evaluative criteria.

In particular, Mill favored an inductivist logical approach, which holds that theories must be justified based on inductive inferences. He went further in arguing that, until there is inductive justification provided for the theory, any additional supplementary consolidation, increased simplicity or analogous situations do not prove useful. Indeed, these additions are meaningless until an inductive justification is provided. The most fundamental tent or starting assumption of Mill’s inductivist logic is the belief in the principle of the ‘uniformity of nature,’ the notion that nature behaves in a law-like and constant manner [[CiteRef::Buchdahl (1971)]].

Mill claims that deductive reasoning is “empty”: it says nothing new about the world. Everything established in the conclusion of a deductive argument must already be present in the premises [[CiteRef::Macleod (2016)]]. Therefore, deductive reasoning does not lead to any new knowledge about the world. Furthermore, Mill is radical in his view that mathematics and geometry---areas that lead to acquisition of genuine knowledge---do not employ deductive reasoning. According to Mill, it only appears that mathematics and geometry use deductive reasoning, but on a deeper level, they are using nothing more than inductive reasoning [[CiteRef::Macleod (2016)]]. The idea that mathematics and geometry ''de facto'' employ inductive reasoning allows him to deny the existence of even this form of knowledge, which Kant and Whewell considered a priori.

The unearthing of causal laws of natural phenomenon can eventually be described using general laws, which are arrived at by means of induction. Similarly, inductions continue and we may find that the law that explained the individual observations’ causal mechanism is, itself, part of a larger law. Of course, this larger, overarching law is also uncovered using induction [[CiteRef::Mill (1974a)]]. This process of scientific reasoning continues, and we discover more and more laws through induction. The laws of nature that induction reveals are, according to Mill, “nothing but the uniformities which exist among natural phenomena” [[CiteRef::Mill (1974a)]], an appeal to the age-old principle of the ‘uniformity of nature.’

Arguing in the same vein as British empiricists such as Locke, Mill advances the idea of tabula rasa---human mind is a clean slate with no preconceived, or innate ideas. This position is consistent with his belief in the non-existence of a priori knowledge, which differentiates him from rationalists, Kant and Whewell. Nevertheless, Mill thinks that human mind is malleable in two respects. First, experiences and observations make their mark on the mind, thus molding it accordingly. Second, background conditions, which vary between different cultures, play a cardinal role in shaping one’s mind. Mill thinks that the tremendous amount of “pliability” [[CiteRef::Mill (1977a)]] exhibited by the human mind has implications for our observations.