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|Historical Context=John Stuart Mill, like his predecessor William Whewell, thought that one of the essential tasks of the philosophy of science was to formulate a method of evaluation for scientific theories. Unlike most philosophers before them, both Whewell and Mill were cognizant that history of science and philosophy of science are intimately connected. Yet, they differed in their approaches for what provides, or is the source of justification for the evaluative criteria for scientific theories and laws. On one hand, Whewell took what could appropriately be termed as the ‘historicist’ approach: he thought that the study of history of science will provides us with the evaluative criteria. Specifically, Whewell’s survey of the history of science led him to conclude that the evaluative criteria are ‘distilled from’ and derive their justification out of the historical record. Hence, Whewell’s somewhat unqualified historicist approach not only ‘finds’ the scientific criteria by studying history of science, but also provides its justification based on scientific history.

Mill disagreed in principle that 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 information that certain regularities have held in the past. Mill made the descriptive claim that scientific inquiry is a search for causal connections---correlations 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 also held that induction is, in principle, 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 they will remain invariable in the future as well. Relatedly, he differed with Whewell on a further point: because scientific theories cannot be said to be invariable due to induction (and as historical record is an inadequate justification), it follows that historical record of science does not equip the theories with unconditionality. Unconditionality of scientific theories could roughly be interpreted as theories that are ‘true’ or not in need of any qualification whatsoever. Mill argued that as the history of science cannot provide justification even for the invariability of scientific theories, by extension it cannot justify unconditionality either.

|Major Contributions=Mill is an empiricist who believes that all our ideas are gained through sense perception. His empiricism is thoroughgoing: There is no source other than experience and observation that provides us with our ideas (SEP, Mill, 2.1). His empiricism is quite radical; in fact, it is phenomenalistic. For him, reality is simply observer-relative sensory phenomenon (Godfrey-Smith, 2003, p.20), which means that all that exists is in the sensations. That is, there is nothing other than sensation that the mind has access to. Mill thinks that external objects (if any) are not perceivable. The only thing we can perceive is “a set of appearances” (System, VIII: 783). Mill’s position that we cannot know anything about how things are in-themselves, but only know how they appear to us is called the “Relativity of Human Knowledge” (Examination, IX: 4).

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 (SEP, Mill, 2.1). Not only does he reject that knowledge of extension, substance and place as a priori, he instead argues that this type of seemingly a priori knowledge is “put together out of ideas of sensation” (Examination, IX: 9). In essence, all of our knowledge, including knowledge that is traditionally thought of as a priori, originates from and is dependent on experience.