But pain and pleasure have two ways of making their appearance in the mind; of which the one has effects very different from the other.

They may either appear in impression to the actual feeling, or only in idea, as at present when I mention them.

It is evident the influence of these upon our actions is far from being equal.

Impressions always actuate the soul, and that in the highest degree; but it is not every idea which has the same effect.

Nature has proceeded with caution in this came, and seems to have carefully avoided the inconveniences of two extremes.
Did impressions alone influence the will, we should every moment of our lives be subject to the greatest calamities; because, though we foresaw their approach, we should not be provided by nature with any principle of action, which might impel us to avoid them.