Quotations about:
    capacity

Do not suppose that if you personally find that something is hard to achieve, it is therefore beyond human capacity; rather, if something is possible and appropriate for human beings, assume that it must also be within your own reach.

[Μή, εἴ τι αὐτῷ σοὶ δυσκαταπόνητον, τοῦτο ἀνθρώπῳ ἀδύνατον ὑπολαμβάνειν, ἀλλ̓ εἴ τι ἀνθρώπῳ δυνατὸν καὶ οἰκεῖον, τοῦτο καὶ σεαυτῷ ἐφικτὸν νόμιζε.]

Marcus Aurelius (AD 121-180) Roman emperor (161-180), Stoic philosopher

Meditations [To Himself; Τὰ εἰς ἑαυτόν], Book 6, ch. 19 (6.19) (AD 161-180) [tr. Hard (2011 ed.)]
    (Source)

(Source (Greek)). Alternate translations:

Do not ever conceive anything impossible to man, which by thee cannot, or not without much difficulty be effected; but whatsoever in general thou canst Conceive possible and proper unto any man, think that very possible unto thee also.
[tr. Casaubon (1634), 6.18]

Because you find a Thing very difficult, don't presently conclude that no Man can master it. But whatever you observe proper, and practicable by Another, believe likewise within your own power.
[tr. Collier (1701)]

If any thing seems exceedingly difficult for you to accomplish, don’t conclude it to be impossible to all men: but rather, if you see any thing possible to man, and a part of his proper work, conclude that you also may attain to it.
[tr. Hutcheson/Moor (1742)]

Do not conclude, because you find a thing difficult, that therefore it is beyond the power of man to perform. But, whatever you see practicable by other men, if it be proper to be done, be assured it is in your power to perform.
[tr. Graves (1792), 6.18]

If a thing is difficult to be accomplished by thyself, do not think that it is impossible for man: but if anything is possible for man and conformable to his nature, think that this can be attained by thyself too.
[tr. Long (1862)]

Because you find a thing very difficult, do not at once conclude that no man can master it. But whatever you observe proper and practicable by another, believe likewise within your power.
[tr. Collier/Zimmern (1887)]

Do not think that what is hard for thee to master is impossible for man; but if a thing is possible and proper to man, deem it attainable by thee.
[tr. Morgan (1894)]

Because your own strength is unequal to the task, do not assume that it is beyond the powers of man; but if anything is within the powers and province of man, believe that it is within your own compass also.
[tr. Rendall (1898)]

If a thing seems to you very difficult to accomplish, conclude not that it is beyond human power. But, if you see that anything is within man’s power, and part of his proper work, conclude that you also may attain to it.
[tr. Hutcheson/Chrystal (1902)]

Because thou findest a thing difficult for thyself to accomplish do not conceive it to be impracticable for others; but whatever is possible for a man and in keeping with his nature consider also attainable by thyself.
[tr. Haines (Loeb) (1916)]

Do not because a thing is hard for you yourself to accomplish, imagine that it is humanly impossible: but if a thing is humanly possible and appropriate, consider it also to be within your own reach.
[tr. Farquharson (1944)]

Because a thing is difficult for you, do not therefore suppose it to be beyond mortal power. On the contrary, if anything is possible and proper for men to do, assume that it must fall within your own capacity.
[tr. Staniforth (1964)]

Do not suppose that, if you find something hard to achieve, it is beyond human capacity; rather, if something is possible and appropriate for man, assume that it must also be within your own reach.
[tr. Hard (1997 ed.)]

Not to assume it's impossible because you find it hard. But to recognize that if it's humanly possible, you can do it too.
[tr. Hays (2003)]

Do not imagine that, if something is hard for you to achieve, it is therefore impossible for any man: but rather consider anything that is humanly possible and appropriate to lie within your own reach too.
[tr. Hammond (2006)]

If something is difficult for you to accomplish, do not then think it impossible for any human being; rather, if it is humanly possible and corresponds to human nature, know that it is attainable by you as well.
[tr. Needleman/Piazza (2008)]

If something is exceedingly hard for you to do, don’t suppose that it is impossible for a human, but if it is something possible and suitable for a human, think that it is within your power as well.
[tr. @aleator (2012)]

If something is hard for you to achieve, do not suppose that it is beyond human capacity; rather, if something is possible and suitable for human beings, consider that it is within your reach too.
[tr. Gill (2013)]