''An Essay Concerning Human Understanding 1690 '' by JohnLocke :

The acts of the mind, wherein it exerts its power over simple ideas, are chiefly these three: 
* Combining several simple ideas into one compound one, and thus all complex ideas are made. 
* 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. 
* 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.

Available online in two volumes: http://www.gutenberg.org/etext/10615 and http://www.gutenberg.org/etext/10616.

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