To command their professors of astronomy to refute their own observations is to command them not to see what they do see and not to understand what they do understand.

Some, merely to contradict what I had said, did not scruple to cast doubt upon things they had seen with their own eyes again and again.

The earth, in fair and grateful exchange, pays back to the moon an illumination similar to that which it receives from her throughout nearly all the darkest gloom of the night.

By denying scientific principles, one may maintain any paradox.

It was granted to me alone to discover all the new phenomena in the sky and nothing to anybody else. This is the truth which neither envy nor malice can supress.

I am certainly interested in a tribunal in which, for having used my reason, I was deemed little less than a heretic. Who knows but men will reduce me from the profession of a philosopher to that of historian of the Inquisition!

Measure what can be measured, and make measureable what cannot be measured.