If he cannot; he here runs in a circle, and gives a synonimous term instead of a definition.
Shall we then rest contented with these two relations of contiguity and succession, as affording a complete idea of causation?
By, no means.

An object may be contiguous and prior to another, without being considered as its cause.

There is a NECESSARY CONNEXION to be taken into consideration; and that relation is of much greater importance, than any of the other two above-mentioned.
Here again I turn the object on all sides, in order to discover the nature of this necessary connexion, and find the impression, or impressions, from which its idea may be derived.

When I cast my eye on the known Qualities of objects, I immediately discover that the relation of cause and effect depends not in the least on them.

When I consider their relations, I can find none but those of contiguity and succession; which I have already regarded as imperfect and unsatisfactory.