It has always struck me that this sentence seems to be particularly thorny for the translators to make clear. This is Bailey:
[500] And if reason is unable to unravel the cause, why those things which close at hand were square, are seen round from a distance, still it is better through lack of reasoning to be at fault in accounting for the causes of either shape, rather than to let things clear seen slip abroad from your grasp, and to assail the grounds of belief, and to pluck up the whole foundations on which life and existence rest.
The different translations I have seen almost never fail to seem awkward, but the meaning seems to be:
It's better to admit that you don't know rather than to admit that there is anything beyond or above the senses that will let you determine the answer without them, or in contradiction of them. Because if you fall for that trap then you'll be totally lost in imaginary traps.
[Edit: Scratch that. More importantly, the sentence that is even harder to translate seems to be at the end of that, because the "senses" should not be described as false. (As it seems to me he often does Brown does a little better):
Bailey: [513] Again, just as in a building, if the first ruler is awry, and if the square is wrong and out of the straight lines, if the level sags a whit in any place, it must needs be that the whole structure will be made faulty and crooked, all awry, bulging, leaning forwards or backwards, and out of harmony, so that some parts seem already to long to fall, or do fall, all betrayed by the first wrong measurements; even so then your reasoning of things must be awry and false, which all springs from false senses.
Brown: So the reason of things must of necessity be wrong and false which is founded upon a false representation of the senses.
Munro: Once more, as in a building, if the rule first applied is wry, and the square is untrue and swerves from its straight lines, and if there is the slightest hitch in any part of the level, all the construction must be faulty, all must be wry, crooked, sloping, leaning forwards, leaning backwards, without symmetry, so that some parts seem ready to fall, others do fall, ruined all by the first erroneous measurements; so too all reason of things must needs prove to you distorted and false, which is founded on false senses.
]