How much more precious is a little humanity than all the rules in the world.

What we see changes what we know. What we know changes what we see.

Every time we teach a child something, we keep him from inventing it himself. On the other hand, that which we allow him to discover for himself will remain with him visible for the rest of his life.

I could not think without writing.

Scientific thought, then, is not momentary; it is not a static instance; it is a process.

It is with children that we have the best chance of studying the development of logical knowledge, mathematical knowledge, physical knowledge, and so forth.

Equilibrium is the profoundest tendency of all human activity.