One common criterion of truth is simple, straightforward pragmatism: if a belief "works," then it must be true. If it doesn't work, then it must be false and should be discarded in favor of something else. This criterion has the distinct advantage of being readily testable — in fact, the principle that beliefs and ideas must be verified before being accepted resonates strongly in scientific circles.
The pragmatic test for truth goes a bit further than just the scientific principle of verification, however. For pragmatists, the very meaning and nature of an idea can only be discerned when it is applied to real-world situations. Ideas which are only in the mind have less substance and less relevance. It is in the actions of our lives that meaning and truth are located, not in idle speculation.
Can False Beliefs Fail?
There is certainly a lot to be said for relying on pragmatism when trying to distinguish between true and false ideas. After all, you can always point to a successful test or project and demonstrate to others the validity of your beliefs. If your ideas weren't true, they couldn't possibly result in such success, right?
Well, maybe they could. The problem with relying heavily or even exclusively upon pragmatism is that it simply isn't the case that only true beliefs "work" and false beliefs don't. It's entirely possible for the success to be the consequence of something other than your belief.
For example, a doctor could prescribe a medication to a sick person and watch an illness disappear — but that doesn't automatically mean that the medication was the cause of the improvement. Maybe it was a change in the patient's diet, or perhaps the patient's immune system finally won out.
Can True Beliefs Fail?
In addition to false belief appearing to "work," true beliefs can also appear to fail. Once again, factors which lie outside your knowledge and control can intervene to cause a project which should succeed to ultimately fail. This happens less often, especially in carefully controlled studies, and as a result this sort of Negative Pragmatism (failed tests point to false ideas) is a bit stronger. Nevertheless, that really only works after rigorous and repeated testing — a single failed test is often not enough to give up on an idea.
The problem here is that the world around us is much more complex than we tend to realize on a conscious level. No matter what we are doing, there are far more factors involved than we usually think of — many of which we just take for granted, like natural laws or our own memories. Some things (like natural laws) are indeed reliable, but others (like the human memory) are not nearly so reliable as we assume.
What Does it Mean for a Belief to "Work"?
Because of this, it can be very difficult to tell whether or not something is "working" at all, much less why. When we attribute something that works to a single belief which we then conclude is true, we are often simplifying matters incredibly. Sometimes this isn't a problem — and we do often have to simplify because, quite frankly, life and nature are just too complex to take in all at once.
However necessary simplification may be, it still introduces a level of uncertainty into our calculations and increases the chances of error. As a consequence, even though pragmatism can be a very practical and useful test for truth, it is still one which needs to be used with caution.