The primitive stages can always be re-established; the primitive mind is, in the fullest meaning of the word, imperishable.

The functional importance of the ego is manifested in the fact that normally control over the approaches to motility devolves upon it. Thus in its relation to the id it is like a man on horse back, who has to hold in check the superior strength of the horse; with this difference, that the rider tries to do so with his own strength while the ego uses borrowed forces.

If there are quarrels between the parents or if their marriage is unhappy, the ground will be prepared in their children for the severest predisposition to a disturbance of sexual development or to neurotic illness.

Knowledge is the intellectual manipulation of carefully verified observations.

Men have gained control over the forces of nature to such an extent that with their help they would have no difficulty exterminating one another to the last man. They know this, and hence comes a large part of their current unrest, their unhappiness and their mood of anxiety.

One must learn to give up momentary, uncertain and destructive pleasure for delayed, restrained, but dependable pleasure.

The facts which have caused us to believe in the dominance of the pleasure principle in mental life also find expression in the hypothesis that the mental apparatus endeavours to keep the quantity of excitation present in it as low as possible or at least to keep it constant.