Skepticism, both a philosophy and movement, questions and doubts the certainty of knowledge. It attempts to dismantle the very foundation of truth or anything that can be said to be objectively true. Beginning with the ancient Greek philosopher Pyrrho, skepticism seeks to dismiss any claim to absolute truth, insisting that beliefs are mere opinions that we tend to exert as truths without evidence. To a Pyrrhonian skeptic, anyone who avows or believes what is not evident is just being dogmatic. Any assertion of how and what things ‘really’ are, void of their appearances to one’s senses, is to assert the non-evident. In other words, things are simply “indeterminable”, meaning one cannot know or determine that things are more this way than they are that way.

Influenced by Pyrrho, Sextus Empiricus – also an ancient Greek philosopher – develops his skeptical arguments through different modes of reasoning. He claims that the sense organs of animals differ from species to species. He argues that because there are fundamental differences among humans in terms of pleasures, desires, passions, differing appearances based on differing circumstances as well as differences in laws and customs, it is impossible to know absolutely and with certainty what the truth really is. Sextus Empiricus uses the modes of “infinite regress” and “circular reasoning” or “reciprocal” to show that there can be no definite description or answer to a question. 

His argument goes like this. Let’s assume that X1 holds because of X2. A skeptic may ask what makes X2 holds. If X2 holds because of X3, and X3 because of X4, and so on perpetually, then we will regress infinitely. And if X2 is supported by X3 and X3 by X1, then we have a situation of reciprocal or circular reasoning. In both cases, we cannot know if X1 is true.

Since it is difficult to arrive at absolute knowledge or truth, Sextus Empiricus suggests we “suspend judgment” and just describe things based on appearances, i.e., as each individual perceives things, and not depend on any claim to objective reality. Hence, he doubts whether anything can be known. But he claims that we can use “criterion’’ to ask questions or derive some conclusion. But the criterion itself will be questioned if infinite regress or reciprocal mode (circularity) is considered.

This really puts us in a difficult spot. Should we just suspend judgment on everything? As human beings we have beliefs or at least we are prone to hold onto our beliefs. Having beliefs is essentially part of what it means to be human. Does it make sense to suspend those beliefs since truth cannot be known with absolute certainty?

St. Augustine of Hippo – North African philosopher and theologian who has been largely appropriated by the Western philosophical tradition – disagrees because, according to him, some things or truths can be known with certainty: you exist, you know that you exist, and you are excited about your existence. But a skeptic may ask: what if you are mistaken? Well, according to Augustine, if you are mistaken, it simply means you actually exist, because anything that does not exist cannot be mistaken. So, you must exist if you are mistaken. Further, since you must exist in order for you to be mistaken, even if you are mistaken, there can be no doubt that you are not mistaken in your knowledge that you exist. It then follows that you are not mistaken in knowing that you know. Just as you know that you exist, you also know that you know. When you are excited about these truths, you may as well add this excitement to the things you know as an equally worthy truth or fact. Since as a human being you are not mistaken about the things you love and desire, therefore, you are not mistaken about your excitement. Even if the things you love are illusions, it is still true that you love illusions.

Therefore, Augustine suggests, knowledge and certainty are possible and can be grasped. Certain truths like a person’s existence, thoughts and feelings are available to herself. In other words, the first thing she knows for certain is about herself, especially her soul.

Beyond the truths of our individual lives, can we know other truths? St. Augustine seems to assert that there are mathematical truths and that they can be known. He denies the skeptical argument that because we all have differing sense organs or that human pleasures and desires vary, so it is difficult to know the truth about a particular thing. Augustine claims that certain things are truths, now or forever, and independent of our bodily senses. For example, the numbers seven and three. There has never been and there will never be a time when 7 and 3 are not 10. Thus, the truth of numbers is unchanging and incorruptible. Regardless of individual sensations or perceptions, this truth always holds, absolutely and necessarily. 

Taking mathematical truths even further: Let us assume we have a circle in which two radii are drawn from the circumference to the center. If we make the points at which the radii meet as close together as we like, that does not change the fact that the only point at which these two lines meet is the center. We can draw the circle or shift the lines anyhow we want, but this is and will be true regardless. Moreover, we know that between two lines no matter how close, we can draw another circle or countless other lines. Thus, this is true and it is and can be known by us. Such truth is not unique to each individual nor does it differ from person to person, for it is knowledge that is available to all.

But for Augustine, there is a kind of truth – “practical truth” – that is as incorruptible as mathematical truths. We may ask: how is it that the beauty of things that last for a single day can be considered as good as the beauty of other things that last for more than a day or forever? Here then is a truth that concerns what is “better” or “best”, one that has practical effects or implications. The best of all things – real or possible – must be the highest good, must attain the desire for actual happiness. Hence, it must be eternal and incorruptible. Anything to the contrary, that one already has, is subject to be taken away from her. And she will not be happy if she chooses less than what is good.

Thus, this is as true as it is practical. Truth is eternal and unchanging just as seven plus three equals ten. For truth to exist, it does not depend on us. Truth is superior and beyond each of us. It is something that we acknowledge no matter how we feel. Skepticism, therefore, is mistaken, and Sextus Empiricus’ modes have not shown that each person has her own version of truth. Truth is common to all and we can know it. Even if we refuse to accept it, it still remains the truth.