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. Shall the despair of success make me assert, that I am here possessed of an idea, which is not preceded by any similar impression? This would be too strong a proof of levity and inconstancy; since the contrary principle has been already so firmly established, as to admit of no farther doubt; at least, till we have more fully examined the present difficulty. We must therefore proceed like those who, being in search of any thing that lies concealed from them, and not finding it in the place they expected, beat about all the neighbouring fields, without any certain view or design, in hopes their good fortune will at last guide them to what they search for. 'Tis necessary for us to leave the direct survey of this question concerning the nature of that _necessary connexion_, which enters into our idea of cause and effect; and endeavour to find some other questions, the examination of which will perhaps afford a hint, that may serve to clear up the present difficulty. Of these questions there occur two, which I shall proceed to examine, viz. First, for what reason we pronounce it _necessary_, that every thing whose existence has a beginning, should also have a cause? Secondly, why we conclude, that such particular causes must _necessarily_ have such particular effects; and what is the nature of that _inference_ we draw from the one to the other, and of the _belief_ we repose in it? I shall only observe before I proceed any farther, that though the ideas of cause and effect be derived from the impressions of reflection as well as from those of sensation, yet for brevity's sake, I commonly mention only the latter as the origin of these ideas; though I desire that, whatever I say of them, may also extend to the former. Passions are connected with their objects and with one another; no less than external bodies are connected together. The same relation then of cause and effect, which belongs to one, must be common to all of them. [2] Part IV. Sect 5. SECTION III. WHY A CAUSE IS ALWAYS NECESSARY.