人类理解研究

All ideas, especially abstract ideas, are naturally weak and ambiguous, and the mind does not grasp them strongly; they are most easily confused with other similar ideas, and after we have used any term, we are apt to imagine that it has a definite idea attached to it, even though it does not have any clear meaning. On the other hand, all impressions, that is, all sensations, whether internal or external, are strong and active; their boundaries are more precise and definite, and in this respect we are not easily caught in error.