2014 UCLA Logic Summer School:
Forcing and Independence in Set Theory

There are certainly moments in any mathematical discovery when the resolution of a problem takes place at such a subconscious level that, in retrospect, it seems impossible to dissect it and explain its origin. Rather, the entire idea presents itself at once, often perhaps in a vague form, but gradually becomes more precise. Since the entire new 'model' M(a) is constructed by transfinite induction on ordinals, the definition of what is meant by saying a is generic must also be given by a transfinite induction. Yet a, as a set of integers, occurs very early in the rank hierarchy of sets, so there can be no question of building a by means of a transfinite induction. The answer is this: the set a will not be determined completely, yet properties of a will be completely determined on the basis of very incomplete information about a. I would like to pause and ask the reader to contemplate the seeming contradiction in the above. This idea as it presented itself to me, appeared so different from any normal way of thinking, that I felt it could have enormous consequences. On the other hand, it seemed to skirt the possibility of contradiction in a very perilous manner. Of course, a new generation has arisen who imbibe this idea with their first serious exposure to set theory, and for them, presumably, it does not have the mystical quality that it had for me when I first thought of it. How could one decide whether a statement about a is true, before we have a? In a somewhat exaggerated sense, it seemed that I would have to examine the very meaning of truth and think about it in a new way.

— Paul Cohen in The Discovery of Forcing, Rocky Mountain Journal of Mathematics, Vol 32, No 4, 2002.