Thinking is a delicate, difficult process, which man cannot perform unless knowledge is his goal, logic is his method, and the judgment of his mind is his guiding absolute. Thought requires selfishness, the fundamental selfishness of a rational faculty that places nothing above the integrity of its own function.
A man cannot think if he places something—anything—above his perception of reality. He cannot follow the evidence unswervingly or uphold his conclusions intransigently, while regarding compliance with other men as his moral imperative, self-abasement as his highest virtue, and sacrifice as his primary duty. He cannot use his brain while surrendering his sovereignty over it, i.e., while accepting his neighbors as its owner and term-setter.
The intensity of a process of thought and of the intellectual effort required varies according to the scope of its content; it varies when one grasps the concept “table” or the concept “justice,” when one grasps that 2 + 2 = 4 or that e = mc2.
~ Found at Ayn Rand Lexicon