The fact that knowledge endlessly recedes as the investigator is about to grasp it is what constitutes at the same time his torment and happiness.

In the philosophic sense, observation shows and experiment teaches.

The true worth of an experimenter consists in his pursuing not only what he seeks in his experiment, but also what he did not seek.

The doubter is a true man of science: he doubts only himself and his interpretations, but he believes in science.

The constancy of the internal environment is the condition for free and independent life: the mechanism that makes it possible is that which assured the maintenance, with the internal environment, of all the conditions necessary for the life of the elements.

A man of science rises ever, in seeking truth; and if he never finds it in its wholeness, he discovers nevertheless very significant fragments; and these fragments of universal truth are precisely what constitutes science.

Priestley [said] that each discovery we make shows us many others that should be made.