Part of the diversity is due to the way in which imagination associates things. Appealing to accounts of imaginative association by Hume and Kames, Gerard notes that in his day it had become clear that imagination does not associate ideas at random. Different people at the time proposed slightly different lists of the basic principles of association; Gerard gives three simple principles of association and three complex principles of association. The simple principles are resemblance, contrariety, and vicinity. The complex principles are co-existence, causation, and order (or organization). The way these principles operate can be easily affected by habit and the passions. Habit has three especially important effects:
(1) "An idea which custom has rendered familiar to us, will be more easily introduced by any present perception, than another idea which is equally related to that perception, but which we are little accustomed to think upon" (EG 126). Thus background experience has an effect.
(2) "[C]ustom renders us more apt to be affected with one of the associating qualities, than with others" (EG 133). So, for instance, if you are a poet and read a great many things in which resemblance is important, this will tend to make resemblances more salient to you than other associative relations.
(3) "Custom not only renders one associating quality more ready to affect us, than other qualities; it likewise renders the same quality readier to operate on the imagination after one particular manner than after another manner" (EG 137). Things may resemble each other in different ways, for instance; but some of these resemblances will be more salient to us because that's the kind of resemblance we've become used to noting.
In the course of talking about various related issues, Gerard has an interesting passage in which he discusses the association between words and ideas. One might think that words and ideas suggest each other equally, but serious regard for experience shows us that this is not so at all: ideas do not suggest words as readily as words suggest ideas. We see this in learning new languages, for instance, where being able to speak and write the language generally requires a much greater facility with the language than simply understanding it. This asymmetry arises because the ideas in question are much more familiar to us than the words. This is actually an interesting point; associative accounts of the imagination usually proceed as if association were symmetrical or else involved asymmetry only in cases where before and after were particularly important. But Gerard's point is that the three effects noted above show that associative principles symmetrical in themselves (like resemblance) can nonetheless have asymmetrical effects due to differences in how familiar we are with the things that are related by them.
The passions also have an effect, one that is especially important for genius in the arts, where excellent representation of the passions can often be important. There are a number of forms these effects can take. Passions are sometimes closely connected with particular ideas, so that getting angry (for instance) makes you think about other things you get angry about. Passions can likewise make associations more salient. They can make the mind oscillate between its present objects and other ideas entirely, as worry or anxiety often will. Passions will often restrict the kinds of ideas that are likely to be brought to mind by association, focusing our attention on those most closely related to the passions themselves. All these have to do with the nature of the ideas suggested through association, but passions can in addition affect the quantity of our ideas, making it difficult to follow long trains of thought. When we do have a long train of thought while in the grip of the passions, this will often be due to the principles of coexistence and causation; the associative principle that the passions most hamper is order, or organization.
All of this ties into the topic of the varieties and kinds of genius, since Gerard's account of the latter is chiefly associative (EG 185):
From the account which has already been given of the principles of association, it is easy to collect, That there is a broad foundation laid in the nature of the human imagination, for great extent and variety of genius. There are many relations of ideas, which fit them for being associated; almost every perception bears some of these relations to many different ideas; habits and the passions multiply and vary the instruments of association: by these means there are innumerable handles by which the imagination may seize such ideas as it has occasion for. Genius has, in some men, great force and compass; but a vigorous construction of the associating principles is sufficient to account for it, however great it may be; for if they be vigorous, any one perception may introduce a great multitude of others, and that by means of many different relations.
The principles of association affect the force and form of genius most clearly in combination, when they come together in various ways; since genius in Gerard's account is great facility with the combination of ideas, this capacity to associate ideas according to many different relations at once is quite central to his account. Each person, however, will tend to favor some of these principles over others, which will lead to their minds going off in different directions even when they start in the same place. Because of this, the kinds of genius can be relatively easily distinguished. Likewise, we can explain why genius in one field may even hamper thought in another; if the fields require very different associating principles to predominate, precisely what gives a person an advantage in one field may give them a disadvantage in another, at least if the advantage is due to a strong habit of association.
Differences in memory will also diversify the kinds of genius. Two aspects of this are particularly notable.
(1) "[T]he peculiar turn of memory will affect genius, by determining, in many instances, the perception from which it sets out, in its investigations or compositions" (EG 273).
(2) "[T]he particular things which are strongly remembered by a person, will directly influence the ideas introduced, as well as the perception from which he sets out" (EG 275).
Judgment will also have an effect. There are judgments of truth and judgments of beauty, for instance. Judgments of truth can consider either matters of fact or relations of ideas, and in each case it can be certain or probable, intuitive or discursive. Judgment of beauty is also called taste, and all the various factors previously introduced in our look at taste in general (sensibility, refinement, correctness, proportion) can affect genius by being stronger or weaker. On Gerard's account, of course, genius does not directly depend on taste, nor does it follow from it directly; but since someone acting out of genius much judge how his or her work is going, judgment is ineliminable from the actual operation of genius.
All these factors contribute to divide genius into different kinds. The most important division is that between the kind of genius most generally appropriate for production and the kind of genius most generally appropriate for inquiry. These have a very different character to them: "A genius for science is formed by penetration; a genius for the arts, by brightness" (EG 322).
The penetrating genius associated with science and philosophy is concerned with truth and explanation. It depends on acuteness, but also on "a capacity of bringing quickly and completely into view, whatever materials are necessary for our present purpose" (EG 324). It involves being able to keep track of many different relations simultaneously. The bright genius associated with the arts, however, is concerned with adornment. It is far more important for the artist to be able to move swiftly among ideas than to take everything into account all at once. Penetration requires a capacity for being most strongly affected by the most strongly associated things; whereas brightness requires a capacity for moving onto less traveled associative by-roads. The great artist gives us something new and unusual in a way that the great scientist generally does not; whereas the great scientist gives us something comprehensive to a degree that great artists usually cannot.
Likewise, different principles of association tend to predominate with each. Penetrating minds tend to be more strongly governed by coexistence and causation. The dominant principle of association for bright minds, on the other hand, tends to be resemblance. This makes sense, if you think about it. Minds that tend strongly to associate ideas according to coexistence and causation will have an advantage in explanation, which is chiefly structured by causation and heavily affected by coexistent conditions. Minds that tend strongly to associate ideas according to resemblance, on the other hand, will tend to have an advantage in description, since description is greatly facilitated by good analogies, metaphors, and images. Further, given what Gerard has already said about habit, it is clear that people will tend to play to their strengths. Someone who handles causation or coexistence well will tend to be more comfortable with tasks that require appeal to causes or concomitant conditions, while someone who handles resemblance well will tend to be more comfortable with tasks that require producing representations (whether verbal or otherwise).
Of course, it's important to keep in mind that all the principles of association are always operative, and all genius is separated from even the most remotely contrary kinds of genius by something like a continuum. Someone whose primary principle of association is causation, like Newton, may have a brilliant subordinate facility with resemblances. Because of this Newton's scientific work has some features reminiscent of poetry, but the kinds of resemblance and analogies that predominant are not the kinds a poet would find salient. They are resemblances of one experiment or phenomenon to other. Likewise, poets, driven chiefly by resemblances, may have excellent subordinate tendencies to take into account causation. Such poets would tend to pick out different causes -- more remote, perhaps, or more subtle, the supplemental nuances of causal explanation -- than those who were primarily driven by causal association itself, which would tend to focus, obviously, on the dominant causal lines. The painter's mind may be highly taken with coexistence; but this principle will likely have a looser hold on his mind than it would on (say) a physicist, and the kinds of coexistence that concern him will likely be somewhat marginal to the physicist's interests. In any case, this all contributes to quality of genius: Newton's greatness as a scientist lies not merely in his penetration but in the fact that he had a very keen power of association by resemblance that was subordinated to it. Gerard describes this dynamic as the sun (the predominant principle of association) shining but its own inherent vigor, while the moon (the subordinate principle) reflects this light depending on how it is situated with respect to the sun.
One important difference between scientific and artistic genius is that scientific genius is more seriously hampered by the interference of the passions, whereas artistic genius can find in the involvement of the passions precisely what it requires. In scientific investigations, the interest in truth means that bias is a serious worry. In artistic production, however, the interest in beauty has the result that bias is not a major concern, and, what is more, since part of what the artist is to trying to do is make things vivid, a sense of the passions and how they operate in a particular context is essential, since few things make ideas brilliantly vivid as the passions do.
Another difference is in how memory functions in each. Scientific genius is best aided precise memory, keeping the connections accurate, whereas artistic genius is most greatly strengthened by vivid memory, keeping the sense of experience intact, regardless of whether the connections are kept in order. And, of course, they will often be concerned with remembering different kinds of things.
Genius for the arts is necessarily more imaginative than genius for the sciences, since genius for the sciences is mostly concerned with collecting materials for judgment, whereas genius for the arts can do a much larger portion of its task purely by imagination. This has led some people to treat imagination as having nothing to do with science or philosophy, and to talk as if 'imaginative works' included only works of art. Further, people often treat the arts as involving nothing but imagination. This is false, however. Genius for the arts may in some sense require more strength and sweep of imagination, but it too needs judgment; and genius for science may require more acuteness and vigor of judgment, but it too requires a considerable amount of imaginative work, and while the soarings of imagination may not be as expansive for it, they are there. This is the foundation, incidentally, why Gerard would not accept Kant's later view that genius is properly concerned only with arts of the beautiful: Kant makes exactly the move that Gerard in the Essay on Genius regards as a serious mistake, treating science as purely a matter of judgment. The difference between scientific ingenuity and artistic ingenuity is a difference arising not from any absolute difference but from different proportions of their common constituents, and, Gerard insists, even in genius for science imagination is the predominant element.
Genius for the arts is also not purely imaginative, because artistic ingenuity can go haywire if it is not corrected by good taste, by a good judgment about whether something is beautiful or striking. This judgment of beauty, again, is taste, and so we are brought back again to our starting point. In the next past in this series we will return to the later editions of the Essay on Taste in order to look at the single most important philosophical question in the theory of taste: What is the standard of taste?