“Theories are like a stairway; by climbing, science widens its horizon more and more, because theories embody and necessarily include proportionately more facts as they advance.”

Science admits no exceptions; otherwise there would be no determinism in science, or rather, there would be no science.

Mediocre men often have the most acquired knowledge. It is in the darker. It is in the darker regions of science that great men are recognized; they are marked by ideas which light up phenomena hitherto obscure and carry science forward.

Experiment is fundamentally only induced observation.

The science of life is a superb and dazzlingly lighted hall which may be reached only by passing through a long and ghastly kitchen.

Our ideas are only intellectual instruments which we use to break into phenomena; we must change them when they have served their purpose, as we change a blunt lancet that we have used long enough.

Now, a living organism is nothing but a wonderful machine endowed with the most marvellous properties and set going by means of the most complex and delicate mechanism.

The eloquence of a scientist is clarity; scientific truth is always more luminous when its beauty is unadorned than when it is tricked out in the embellishments with which our imagination would seek to clothe it.

In science, the best precept is to alter and exchange our ideas as fast as science moves ahead.

We must remain, in a word, in an intellectual disposition which seems paradoxical, but which, in my opinion, represents the true mind of the investigator. We must have a robust faith and yet not believe.

True science teaches us to doubt and, in ignorance, to refrain.

Those who do not know the torment of the unknown cannot have the joy of discovery.

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

Obervation is a passive science, experimentation is an active science.

The investigator should have a robust faith – and yet not believe.

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.

If I had to define life in a single phrase, I should clearly express my thought of throwing into relief one characteristic which, in my opinion, sharply differentiates biological science. I should say: life is creation.

One must feel sorry for those who have strange tastes, but never insult them. Their wrong is Nature’s too; they are no more responsible for having come into the world with tendencies unlike ours than are we for being born bandy-legged or well-proportioned.

Man is distinguished from the brute animals in proportion as thought prevails over sense: but in the healthy processes of the mind, a balance is constantly maintained between the impressions from outward objects and the inward operations of the intellect:–for if there be an overbalance in the contemplative faculty.

We care what happens to people only in proportion as we know what people are.

I drink the wine of aspiration and the drug of illusion. Thus I am never dull.

Idealism increases in direct proportion to one’s distance from the problem.

Don’t begin with proportion. Only prigs do that. Let proportion come in as a last resource, when the better things have failed.

I make the proportion of one of these small Water-creatures to a Cheese-mite, to be like that of a Bee to a Horse:

Declare Church and State forever separate and distinct, but each free within their proper spheres, and that all church property shall bear its own proportion of taxation.