"We are to admit no more causes of natural things than such as are both true and sufficient to explain their appearances."

"To every action there is always opposed an equal reaction."

"The whole difficulty of philosophy seems to consist in this—from the phenomena of motions to investigate the forces of nature, and then from these forces to demonstrate the other phenomena."

"To explain all nature is too difficult a task for any one man or even for any one age."

"The attractive force of the earth acts to the greatest distance, and is observed in the fall of the moon, which is continually drawn towards the earth."