All credibility, all good conscience, all evidence of truth come only from the senses.

Whatever theory you hold has to touch something you can see, hear or handle eventually, or it floats free and stops being checkable at all. Elegant reasoning carries you a long way from anything observable, and after a while the elegance itself becomes the reason you believe it. Contact with the concrete is what keeps thinking honest.

You need not accept that the senses are the only source of anything true to make use of the warning. Take one belief you hold firmly about your work or your relationships, and ask what you have actually observed that would count against it. If nothing could, it is not resting on evidence.