But besides this, there are many original sources of pleasure in the study of the sciences: as, First, From the many instances of uniformity with variety: Secondly, From the marvellous and seemingly impossible which occur in all parts of knowledge: Thirdly, From the great advantages respecting human life, which accrue to mankind in general from the pursuit of knowledge, also from the honours, riches, &c. which are the rewards conferred upon particular persons that are eminent; Lastly, From the numerous connexions of truth of all kinds with those most amiable and important doctrines, which religion, natural and revealed, teaches us. And when these pleasures, in the several subordinate kinds and degrees, have been sufficiently associated with the favourite study, they render it at last pleasant in itself, as we usually term it, i.e. these several particular pleasures coalesce into a single general one, in which the compounding parts cannot be discerned separately from each other, and which consequently appears to have no relation to its several compounding parts; unless when by a particular attention to, and examination of, what passes in our minds, we lay hold of the last compounding parts before their entire coalescence, or reason upon the causes of these pleasures, by comparing their growth, and the changes made in them, with the concomitant circumstances. Thus if it be observed as a general fact, that persons grow fond of particular studies, remarkably after having received some great present advantage, or hope of a future one from them, we may reasonably presume, that the pleasure which they take in these studies, is in part derived from this source, even though it cannot be felt to arise from it explicitly.
The copiousness and quickness of the invention being principal requisites for the cultivation of the arts and sciences with success, I will say something concerning invention here, my subject being now sufficiently opened for that purpose.
Invention then may be defined the art of producing new beauties in works of imagination, and new truths in matters of science. And it seems to depend, in both cases, chiefly upon these three things. First, A strong and quick memory: Secondly, An extensive knowledge in the arts and sciences; and particularly in those that are contiguous to, or not far distant from, that under consideration: and, Thirdly, The habit of forming and pursuing analogies, the deviations from these and the subordinate analogies in many of these first deviations, &c. &c.
First, A strong and quick memory is necessary, that so the ideas of the poet or philosopher may depend upon, and be readily suggested by, each other.
Secondly, He must have a large stock of ideas for the purposes of figures, illustrations, comparisons, arguments, motives,
