Optional Quotes (3 quotes)
Getting to the summit is optional, getting down is mandatory.
…...
The laws and conditions of the production of wealth partake of the character of physical truths. There is nothing optional or arbitrary in them ... It is not so with the Distribution of Wealth. That is a matter of human institution solely. The things once there, mankind, individually or collectively, can do with them as they like.
Principles of Political Economy (1848), Book 2, 199.
Truth in politics is optional — Truth in engineering is mandatory.
From 'Igor Sikorsky the Aviation Pioneer Speaks', Igor I. Sikorsky Historical Archives, sikorskyarchives.com.
In science it often happens that scientists say, 'You know that's a really good argument; my position is mistaken,' and then they would actually change their minds and you never hear that old view from them again. They really do it. It doesn't happen as often as it should, because scientists are human and change is sometimes painful. But it happens every day. I cannot recall the last time something like that happened in politics or religion.
(1987) -- 
