Because an idea lives on after the death of the person.

Ideas, unlike solid structures, do not perish. They remain immortal, immaterial and everywhere, like all Divine things. Ideas are a golden, savage landscape that we wander unaware, without a map. Be careful: in the last analysis, reality may be exactly what we think it is.

A man may die, nations may rise and fall, but an idea lives on. Ideas have endurance without death.

The world for the most part is ruled by the tomb, and the living are tyrannized over by the dead. Old ideas, long after the conditions under which they were produced have passed away, often persist in surviving. Many are disposed to worship the ancient—to follow the old paths, without inquiring where they lead, and without knowing exactly where they wish to go themselves.

One individual may die for an idea, but that idea will, after his death, incarnate itself in a thousand lives.

Some ideas are born as you write then down, others become dead.

The thought that one unifying idea should continue forever is simply not realistic and therefore not to be hoped for, but I think that for quite a number of years still, perhaps if I am lucky to the end of my life, because I would hate to see that stop in my lifetime, those questions will become very active and still somewhat separate, as different branches of learning become accustomed to them. I cannot imagine that this idea would vanish, not because I am so proud of what I've been doing all my life, but because this is not an artificial thought coming from nowhere in no time and vanishing again rapidly in no time. It has in every one of its manifestations profound roots in the history of the various sciences and the various manners of human enterprise and those roots will not be broken. The continuity of these thoughts will continue, and if any substitute comes, if any other name comes, which is possible, the ideas will remain.

There are dead thoughts and there are living thoughts. A dead thought has been compared to a stone which one may plant in the soil. Nothing will come out. A living thought is like a seed. In the process of thinking, an answer without a question is devoid of life. It may enter the mind; it will not penetrate the soul. It may become a part of one’s knowledge; it will not come forth as a creative force.

Dead people, dead ideas and supposedly dead moments are never really dead and they shape every moment of our lives. We ignore them and this makes them powerful.

We are accustomed to speak of ideas as reproduced, as passed from mind to mind, as similar or dissimilar to one another, and, in short, as if they were substantial things; nor can any reasonable objection be raised to such expressions. But taking the word "idea" in the sense of an event in an individual consciousness, it is clear that an idea once past is gone forever, and any supposed recurrence of it is another idea. These two ideas are not present in the same state of consciousness, and therefore cannot possibly be compared.

Ideas become prominent and then fade.

Science is a cemetery of dead ideas, even though life may issue from them.

There were certain basic things, perhaps—the very earth, itself—which existed through every point in time, holding a sort of limited eternity to provide a solid matrix. And the dead—the dead and fabricated—stayed in the past as ghosts. The fence posts and the wire strung on them, the dead trees, the farm buildings, and the bridge were shadows of the present persisting in the past. Persisting, perhaps, reluctantly, because since they had no life they could not move along. They were bound in time and stretched through time and they were long, long shadows.
He was, he realized with a shock, the only living thing existing in this moment on this earth. He and nothing else.

Still I can't get it out of my mind what a discrepancy there is between ideas and living. A permanent dislocation, though we try to cover the two with a bright awning. And it won't go. Ideas have to be wedded to action; if there is no sex, no vitality in them, there is no action. Ideas cannot exist alone in the vacuum of the mind. Ideas are related to living: liver ideas, kidney ideas, interstitial ideas, etc. If it were only for the sake of an idea Copernicus would have smashed the existent macrocosm and Columbus would have foundered in the Sargasso Sea. The aesthetics of the idea breeds flowerpots and flowerpots you put on the window sill. But if there be no rain or sun of what use putting flowerpots outside the window?