For the man of education will seek exactness so far in each subject as the nature of the thing admits, it being plainly much the same absurdity to put up with a mathematician who tries to persuade instead of proving, and to demand strict demonstrative reasoning of a Rhetorician.
[tr. Chase (1847), ch. 1]

It is the mark of an instructed mind to rest satisfied with the degree of precision which the nature of the subject admits and not to seek exactness where only an approximation is possible.
[Source (1852) - earliest version of this frequent translation I found.]

A man who has been well trained will not in any case look for more accuracy than the nature of the matter allows; for to expect exact demonstration from a rhetorician is as absurd as to accept from a mathematician a statement only probable.
[tr. Williams (1869)]

For an educated person will expect accuracy in each subject only so far as the nature of the subject allows; he might as well accept probable reasoning from a mathematician as require demonstrative proofs from a rhetorician.
[tr. Welldon (1892), ch. 2]

For it is the mark of an educated man to require, in each kind of inquiry, just so much exactness as the subject admits of: it is equally absurd to accept probable reasoning from a mathematician, and to demand scientific proof from an orator.
[tr. Peters (1893)]

For it is the mark of an educated mind to expect that amount of exactness in each kind which the nature of the particular subject admits. It is equally unreasonable to accept merely probable conclusions from a mathematician and to demand strict demonstration from an orator.
[tr. Rackham (1934)]

For it is characteristic of a well-educated person to look for the degree of exactness in each kind of investigation that the nature of the subject itself allows. For it is evident that accepting persuasive arguments from a mathematician is like demanding demonstrations from a rhetorician.
[tr. Reeve (1948)]

For a well-schooled man is one who searches for that degree of precision in each kind of study which the nature of the subject at hand admits: it is obviously just as foolish to accept arguments of probability from a mathematician as to demand strict demonstrations from an orator.
[tr. Ostwald (1962)]

For it is the mark of an educated man to seek as much precision in things of a given genus as their nature allows, for to accept persuasive arguments from a mathematician appears to be [as improper as] to demand demonstrations from a rhetorician.
[tr. Apostle (1975)]

For it is the mark of the trained mind never to expect more precision in the treatment of any subject than the nature of that subject permits; for demanding logical demonstrates from a teacher of rhetoric is clearly about as reasonable as accepting mere plausibility from a mathematician.
[tr. Thomson/Tredennick (1976)]

Since it is the mark of an educated person to look in each area for only that degree of accuracy that the nature of the subject permits. Accepting persuasive arguments from a mathematician is like demanding demonstrations from a rhetorician.
[tr. Crisp (2000)]

For it is the mark of an educated person to search for the same kind of clarity in each topic to the extent that the nature of the matter accepts it. For it is similar to expect a mathematician to speak persuasively or for an orator to furnish clear proofs!
[tr. @sentantiq (2018)]

For it belongs to an educated person to seek out precision in each genus to the extent that the nature of the matter allows: to accept persuasive speech from a skilled mathematician appears comparable to demanding demonstrations from a skilled rhetorician.
[tr. Bartlett/Collins (2011)]