Therefore, to clear up the nature of these qualities, it may be necessary to explain the nature of pain and pleasure on which they depend.

A man who suffers under violent bodily pain, (I suppose the most violent, because the effect may be the more obvious,) I say a man in great pain has his teeth set, his eyebrows are violently contracted, his forehead is wrinkled, his eyes are dragged inwards, and rolled with great vehemence, his hair stands on end, the voice is forced out in short shrieks and groans, and the whole fabric totters.

Fear or terror, which is an apprehension of pain or death, exhibits exactly the same effects, approaching in violence to those just mentioned, in proportion to the nearness of the cause, and the weakness of the subject.

This is not only so in the human species: but I have more than once observed in dogs, under an apprehension of punishment, that they have writhed their bodies, and yelped, and howled, as if they had actually felt the blows.

From hence I conclude, that pain and fear act upon the same parts of the body, and in the same manner, though somewhat differing in degree: that pain and fear consist in an unnatural tension of the nerves; that this is sometimes accompanied with an unnatural strength, which sometimes suddenly changes into an extraordinary weakness; that these effects often come on alternately, and are sometimes mixed with each other.