I was once present at the lectures of a profound philosopher, a man really skilled in the science which he professed, who having occasion to explain the terms opacum and pellucidum, told us, after some hesitation, that opacum was, as one might say, opake, and that pellucidum signified pellucid.  Such was the dexterity with which this learned reader facilitated to his auditors the intricacies of science; and so true is it, that a man may know what he cannot teach.

Boerhaave complains, that the writers who have treated of chymistry before him, are useless to the greater part of students, because they presuppose their readers to have such degrees of skill as are not often to be found.  Into the same errour are all men apt to fall, who have familiarized any subject to themselves in solitude:  they discourse, as if they thought every other man had been employed in the same inquiries; and expect that short hints and obscure allusions will produce in others the same train of ideas which they excite in themselves.

Nor is this the only inconvenience which the man of study suffers from a recluse life.  When he meets with an opinion that pleases him, he catches it up with eagerness; looks only after such arguments as tend to his confirmation; or spares himself the trouble of discussion, and adopts it with very little proof; indulges it long without suspicion, and in time unites it to the general body of his knowledge, and treasures it up among incontestable truths:  but when he comes into the world among men who, arguing upon dissimilar principles, have been led to different conclusions, and being placed in various situations, view the same object on many sides; he finds his darling position attacked, and himself in no condition to defend it:  having thought always in one train, he is in the state of a man who having fenced always with the same master, is perplexed and amazed by a new posture of his antagonist; he is entangled in unexpected difficulties, he is harassed by sudden objections, he is unprovided with solutions or replies; his surprise impedes his natural powers of reasoning, his thoughts are scattered and confounded, and he gratifies the pride of airy petulance with an easy victory.

It is difficult to imagine, with what obstinacy truths which one mind perceives almost by intuition, will be rejected by another; and how many artifices must be practised, to procure admission for the most evident propositions into understandings frighted by their novelty, or hardened against them by accidental prejudice; it can scarcely be conceived, how frequently, in these extemporaneous controversies, the dull will be subtle, and the acute absurd; how often stupidity will elude the force of argument, by involving itself in its own gloom; and mistaken ingenuity will weave artful fallacies, which reason can scarcely find means to disentangle.

In these encounters the learning of the recluse usually fails him:  nothing but long habit and frequent experiments can confer the power of changing a position into various forms, presenting it in different points of view, connecting it with known and granted truths, fortifying it with intelligible arguments, and illustrating it by apt similitudes; and he, therefore, that has collected his knowledge in solitude, must learn its application by mixing with mankind.