Every author does not write for every reader; many questions are such as the illiterate part of mankind can have neither interest nor pleasure in discussing, and which, therefore, it would be an useless endeavour to level with common minds, by tiresome circumlocutions or laborious explanations; and many subjects of general use may be treated in a different manner, as the book is intended for the learned or the ignorant. Diffusion and explication are necessary to the instruction of those who, being neither able nor accustomed to think for themselves, can learn only what is expressly taught; but they who can form parallels, discover consequences, and multiply conclusions, are best pleased with involution of argument and compression of thought; they desire only to receive the seeds of knowledge which they may branch out by their own power, to have the way to truth pointed out, which they can then follow without a guide. The Guardian directs one of his pupils, "to think with the wise, but speak with the vulgar." This is a precept specious enough, but not always practicable. Difference of thoughts will produce difference of language. He that thinks with more extent than another will want words of larger meaning; he that thinks with more subtilty will seek for terms of more nice discrimination; and where is the wonder, since words are but the images of things, that he who never knew the original should not know the copies? Yet vanity inclines us to find faults any where rather than in ourselves. He that reads and grows no wiser, seldom suspects his own deficiency; but complains of hard words and obscure sentences, and asks why books are written which cannot be understood? Among the hard words which are no longer to be used, it has been long the custom to number terms of art. "Every man," says Swift, "is more able to explain the subject of an art than its professors; a farmer will tell you, in two words, that he has broken his leg; but a surgeon, after a long discourse, shall leave you as ignorant as you were before." This could only have been said, by such an exact observer of life, in gratification of malignity, or in ostentation of acuteness. Every hour produces instances of the necessity of terms of art. Mankind could never conspire in uniform affectation; it is not but by necessity that every science and every trade has its peculiar language. They that content themselves with general ideas may rest in general terms; but those, whose studies or employments force them upon closer inspection, must have names for particular parts, and words by which they may express various modes of combination, such as none but themselves have occasion to consider.