John Locke was the first important proponent of Empiricism in 17th century England. Locke was a medical doctor by training and attempted to work out a theory of knowledge in terms of sensory experience. Locke demonstrated how various concepts or ideas come from different types of experiences and he was insistent that we possess no innate ideas, in contradistinction to the Rationalists.
Locke's most important work is his Essay concerning Human Understanding.
Locke used the word "idea" in a very general sense to refer to everything that the individual thinks or perceives or the entirety of consciousness. Locke described ideas as either Simple Ideas or Complex Ideas.
Simple ideas are those, which cannot be analyzed into anything simpler and he further categorizes simple ideas into two types: the objective and the subjective. Objective simple ideas include primary qualities whereas the subjective simple ideas include secondary qualities.
The primary qualities include such things as number, figure, extension, motion, and solidity. These qualities belong to the body being observed and cannot be separated from the particular body.
The secondary qualities include such things as color, odor, taste, and temperature. These are subjective sensations of the individual who perceives them, the taste of sugar, the smell of a rose.
Memory is the basis on which complex ideas are formed. Simple ideas are not instantaneous but leave an impression in the mind and consequently them can be combined or assisted with other ideas. The modes, the notions of substance and relation are complex ideas and result from the associative activity of the mind.
For Locke, all knowledge and ideas come through sensory experience. No person possesses only innate ideas. According to Locke, the indvidual is born tabula rasa or with a blank slate. At birth each individual mind is a clean white tablet upon which the experiences of life are written and consequently the individual gains knowledge. Locke proposed that there are two basic sources for Ideas: direct sensory experience and reflection upon those experiences.
Regarding the Idea of God Locke acknowledged that we do not acquire this knowledge through sensory experience. He argued that first, we now that we exist and that this cannot be doubted. Second, we know that "nothing" cannot produce "something" and since we exist, there must have been a "something" (i.e. God) who creates all that exists.
Locke is the first, or so it seems, to begin questioning the cognitive faculty, which was regarded so reverently by the rationalists. The distrust of the reason, which begins with Locke, reaches its climax in the skepticism of David Hume and also requires Kant to question the validity of rational knowledge.
