Berkeley begins the modern idealist tradition with his complete rejection of material as metaphysical substance. Working from Locke, Berkeley’s immaterialism does not deny the existence of objects per-se, but that existence has become contingent on perceptions (of ideas or state-of-mind) by a subject. The subject, referred to as spirit, is analogous to the mind and “active” in the sense of possessing will/judgment and the aperception of ideas. This is to say that there are no mind-independent things which leads to a fully subjective and restricted world-view; the existence of other spirits can only be known through the ideas that they incite in our minds. His moral philosophy is also contrarian to that of Locke; passive-obedience is a negative moral duty to the government without exceptions.
Hume both extended and finalized many of the issues that were contained in Locke and Hume’s empiricism. He’s most known for formulating the problem of induction or the inference of the unknown from known events. Underlying all of the induction is the assumption of uniformity or the regularity in nature; the observations that one makes thus far persists in both time and space outside that window. Such an assumption cannot be demonstrated as one can easily conceive of a world that is non-uniform. Neither can a probable argument towards its validity be meaningful as this is the very question that induction seeks to answer; logic/reason cannot be used to justify this hypothesis.
Hume’s notion of causality relates his conception of induction to man’s psychology. Here, causality is always preceded by the impressions of two conjoined events serialized in time. With repeated apprehensions, ideas or mental states of these impressions are formed such that the latter idea becomes associated with the former impression. The mind, by its necessity to determine relations, cannot help by make the leap to causation from its conditioning (Pavlov later demonstrates this in the 19th century). Related is his conception of the self as a mere bundle of perceptions; we have difficulty experiencing impressions of our self but instead have chains of perceptions that follow by necessity.
Hume’s ethics make an important distinction between reason/passion, is-ought, and moral responsibility in a compatibility view of free-will. Reason is always a slave (to serve) to passion in the sense that knowledge does not compel the will as does feelings/sentiments do. This follows from the formulation of the “is-ought” problem where no amount of descriptive statements can ever deduce a prescriptive statement. His compatablist’s free-will holds that while necessity binds actions to motives, it is the will that must have choice for otherwise the actions would be judged as only given to chance which non-regular.