I doubt not but these consequences will at first sight be received without difficulty, as being evident deductions from principles, which we have already established, and which we have often employed in our reasonings. This evidence both in the first principles, and in the deductions, may seduce us unwarily into the conclusion, and make us imagine it contains nothing extraordinary, nor worthy of our curiosity. But though such an inadvertence may facilitate the reception of this reasoning, it will make it be the more easily forgot; for which reason I think it proper to give warning, that I have just now examined one of the most sublime questions in philosophy, viz. that concerning the power and efficacy of causes; where all the sciences seem so much interested. Such a warning will naturally rouze up the attention of the reader, and make him desire a more full account of my doctrine, as well as of the arguments, on which it is founded. This request is so reasonable, that I cannot refuse complying with it; especially as I am hopeful that these principles, the more they are examined, will acquire the more force and evidence. There is no question, which on account of its importance, as well as difficulty, has caused more disputes both among antient and modern philosophers, than this concerning the efficacy of causes, or that quality which makes them be followed by their effects. But before they entered upon these disputes, methinks it would not have been improper to have examined what idea we have of that efficacy, which is the subject of the controversy. This is what I find principally wanting in their reasonings, and what I shall here endeavour to supply. I begin with observing that the terms of EFFICACY, AGENCY, POWER, FORCE, ENERGY, NECESSITY, CONNEXION, and PRODUCTIVE QUALITY, are all nearly synonymous; and therefore it is an absurdity to employ any of them in defining the rest. By this observation we reject at once all the vulgar definitions, which philosophers have given of power and efficacy; and instead of searching for the idea in these definitions, must look for it in the impressions, from which it is originally derived. If it be a compound idea, it must arise from compound impressions. If simple, from simple impressions.