Certainty is the confidence that something is true. Sometimes, certainty is distinguished from certitude. Certainty is objective, but certitude is subjective. A first principle or self-evident statement is objectively certain, whether a person is sure about it or not. Certitude involves a knower’s assent to that which is certain; it is a subjective acceptance of what is objectively so. In common usage the terms are used interchangeably. The difference is that certainty exists where there is objective reasons or evidence that are commensurate to the degree of certainty claimed. With certitude, however, there need to be a commensurate degree of objective reasons or evidence for the degree one possesses.

Spiritual (Supernatural) Certainty-
If we grant the theist God’s existence, he could give supernatural assurance that something is true. Likewise, if God speaks directly to a person (for example, Abraham in Genesis 22), then that person could have a spiritual certainty that transcends other kinds of certainty because it comes directly from God. Those who have had direct mystical experiences of God such as Paul describes in 2 Corinthians 12, have this kind of certainty. It would be greater than any other kind of certainty, since an omniscient being is its guarantor and omniscience cannot be wrong. As to how or whether such assurance actually exists apart from a supernatural act is a moot point among theologians, although many classical apologists and others argue that it does.

Logical Certainty – Logical certainty is found largely in mathematics and proof. This kind of certainty is involved where the opposite would be a contradiction. Something is certain in this sense when there is no logical possibility it could be false. Since mathematics is reducible to logic it fits in this category. It is found in statements such as 2+2 =4. It is also found in tautologies or statements that are true by definition: All circles are round and no triangle is square.

Metaphysical Certainty- There are, however, some other things of which we can be absolutely certain that are not statements empty of content. For example, I know for certain that I exist. This is undeniably so, since I cannot deny my existence without existing to make the denial. First principles can be known for certain, since the subject and the predicate say the same thing: “Being exists”; “Nonbeing is not Being.” ‘Nonbeing cannot produce Being” is also certain, since produce implies an existing producer.

Moral Certainty-Moral certainty exists where the evidence is so great that the mind lacks any reason to veto the will to believe it is so. One rests on moral certainty with complete confidence. Of course, there is a logical possibility that things of which we are morally certain are false. However, the evidence is so great there is no reason to believe it is false. In legal terms this is what is meant by "beyond a reasonable doubt.”

Practical Certainty (High Probability) - Practical certainty is not as strong as moral certainty. Persons claim to be certain about things they believe have a high probability of truth. One may be certain she had breakfast today, without being able to prove it mathematically or metaphysically. It is true unless something changed her perception, so that she was deluded into thinking she ate breakfast. It is possible to be wrong about these matters.