There are few certainties (see: Fallibilism), and to believe there are many tends to be a mark of stupidity, ignorance, or fanaticism. But there are some certainties, notably of a mathematical and logical nature, which may be used to infer others, and also may be used to infer mere probabilities.

And it is interesting to note something many miss that yet is quite fundamental: Every human being may be quite certain that there is much he or she is not certain of. Indeed, these are a human beings greatest certainties: The uncertainties he or she knows oneself to have.

Also, to infer any contingent statement (one that is neither certainly true nor certainly false) one needs to accept some contingent statement, if only hypothetically, provisionally, or until one has better evidence.

It should be noted that everyone - who lives in some society, at least - accepts at least pragmatically or hypothetically many statements as certain that are not really certain in a mathematical or philosophical sense, but without which life in that society is hardly possible.

These pragmatic or hypothetical certainties, as they were just styled, come in many kinds and qualities, varying from practical, legal or moral ones, to scientific or religious certainties.

Four useful moral rules that relate to (un)certainties are:

(1) One's search for evidence for the statements one believes in should be proportional to the importance one attributes to them.
(2) It is almost certainly morally wrong to use violence for things one knows one is not certain of.
(3) "It is wrong always, everywhere, and for anyone, to believe anything upon insufficient evidence" (Clifford's dictum)
(4) It is always right to try to think rationally and try to act reasonably. 

Finally, it should be noted that anybody who is religious feels or should feel morally certain that anybody who has a different religion is mistaken - from which it at least plausibly follows that religious certainties are almost certainly delusions, whereas it is certain that untold many millions have been murdered because of religious fanaticism.