The object of hope is an apparent good; the object of fear, an apparent evil. Whence the hoped-for good that is expected to come to us we never perceive with security; for if we so perceived it, it would then be certain, and our expectation would more properly be called not hope, but joy. Even the most insubstantial arguments are sufficient for hope. Yea, even what the mind cannot truly conceive can be hoped for, if it can be expressed. Similarly, anything can be feared even though it be not conceived of, provided that it is commonly said to be terrible, or if we should see many simultaneously fleeing; for, even though the cause be unknown, we ourselves also flee, as in those terms that are called panic-terrors.

On Man 12.4