Reply when questioned on the safety of the polio vaccine he developed: It is safe, and you can't get safer than safe. - Jonas Salk

Some people are constructive, if you like. Others are destructive. It's this diversity in humankind that results in some making positive contributions and some negative contributions. It's necessary to have enough who make positive contributions to overcome the problems of each age.

There are three stages of truth. First is that it can't be true, and that's what they said. You couldn't immunize against polio with a killed-virus vaccine. Second phase: they say, "Well, if it's true, it's not very important. And the third stage is, 'Well, we've known it all along."

The evolvers are people who cause things to change. The maintainers of the status quo do everything to keep things from changing. And, there I see differences of perception. Differences in vision. Differences in interpretation, and differences in temperament, in personality. The number of evolvers are much fewer than the maintainers of the status quo. And amongst the evolvers, there are some who are initiators, some who go along with what other people recognize to be new or different.