The acts of the mind, wherein it exerts its power over simple ideas, are chiefly these three:
1. Combining several simple ideas into one compound one, and thus all complex ideas are made.
2. The second is bringing two ideas, whether simple or complex, together, and setting them by one another so as to take a view of them at once, without uniting them into one, by which it gets all its ideas of relations.
3. The third is separating them from all other ideas that accompany them in their real existence: this is called abstraction, and thus all its general ideas are made.
An Essay Concerning Human Understanding
John Locke (1690)
(Found in epigraph to the book Structure and Interpretation of Computer Programs )
In Book II of the Essay, Locke gives his positive account of how we acquire the materials of knowledge. Locke distinguishes a variety of different kinds of ideas in Book II. Locke holds that the mind is a tabula rasa or blank sheet until experience in the form of sensation and reflection provide the basic materials—simple ideas—out of which most of our more complex knowledge is constructed. While the mind may be a blank slate in regard to content, it is plain that Locke thinks we are born with a variety of faculties to receive and abilities to manipulate or process the content once we acquire it. Thus, for example, the mind can engage in three different types of action in putting simple ideas together. The first of these kinds of action is to combine them into complex ideas. Complex ideas are of two kinds, ideas of substances and ideas of modes. Substances are independent existences. Beings that count as substances include God, angels, humans, animals, plants and a variety of constructed things. Modes are dependent existences. These include mathematical and moral ideas, and all the conventional language of religion, politics and culture. The second action which the mind performs is the bringing of two ideas, whether simple or complex, by one another so as to take a view of them at once, without uniting them. This gives us our ideas of relations (II.12.1, N: 163). The third act of the mind is the production of our general ideas by abstraction from particulars, leaving out the particular circumstances of time and place, which would limit the application of an idea to a particular individual. In addition to these abilities, there are such faculties as memory which allow for the storing of ideas.