Man is a rational animal. If one knew only the definition and had never met a man, one would assume that a rational animal meant a reasonable animal. In fact we know that man is, just as often, unreasonable. The possession of reason, which distinguishes him from the lower animals, means that he can act reasonably as they cannot, but also unreasonably, as they cannot.
The animals, not having reason, cannot misuse it. Man has it, can misuse it, does misuse it. ... Man is endlessly ingenious in discovering ways of misusing his reason. The commonest way, perhaps, is to leave it unused. Most of us would rather not think at all when any effort is involved. The use of the body is easy, and promises pleasure. The use of the mind is difficult and holds out no such promises. So man is always trying to by-pass the use of the mind.
He thinks with reluctance, which makes him a slave to habit. He thinks with the will, which makes him a slave to desire. He thinks with the imagination, which makes him a slave to slogans. Not using the full power of his mind, he loses perspective. Things closest -- that is, closest to the body’s power to respond to them -- loom biggest. ...
One result of all these ingenious ways of avoiding the use of the mind is that man is intensely gullible: offer him happiness, and all his defenses are down. And the trouble is that man is not consistent: you cannot even rely upon him to act unreasonably: for he is damaged, but not wholly; and he is free. ... The wandering mind can concentrate, the tired will can make a stand, the thrusting self is capable of supreme sacrifice. You never know when. There is that element in man which makes him incalculable, even to himself” - Frank Sheed (1897-1981), Society and Sanity (pp. 55-58).