What children can do with the assistance of others might be in some sense even more indicative of their mental development than what they can do alone

The child begins to perceive the world not only through his [or her] eyes but also through his [or her] speech

It may be said that the basic characteristic of human behavior in general is that humans personally influence their relations with the environment and through that environment personally change their behavior, subjugating it to their control.

Language is the tool of the tools

There is reason to believe that voluntary activity, more than highly developed intellect, distinguishes humans from the animals which stand closest to them.

Thought is not merely expressed in words, it comes into existence through them