Tag Archives: logic

In Defense of Self Evidence

A self evident proposition is a proposition that any rational person will agree is certainly true upon grasping it. In this post, I will demonstrate that there are self evident propositions.

The strongest argument for the claim that there are self evident propositions is to just list several of them so that the reader can see that some propositions are really self evident. Here are three:

  • There is an objective reality.
  • A thing is identical to itself.
  • I am conscious.

I will explain each of these.

“There is an objective reality” means that there is a fact of the matter about something. For example, if I say that we landed on the Moon, and someone else denies that we landed on the Moon, there is a fact of the matter about which of us is right. The fact of the matter about whether we landed on the Moon is one component of the objective reality in which we live.

The existence of an objective reality is contained in every experience we have. When I experience the fact that my alarm clock is red, I can see immediately that my alarm clock really exists, that it really has certain properties like redness, and that its existence and properties are independent of my mind. This experience shows me directly that there is an objective reality.

“A thing is identical to itself” means that a thing is what it is. A tree is a tree, and has all of the properties required for being a tree. Symbolically, this is often expressed “A is A,” where A refers to anything that exists.

This proposition, called the law of identity, is contained in every thought and every observation we have. When you observe a computer, you also observe that that computer is a computer. When you think of the number one, you can see that the number one is the number one. This is one of the undeniable foundations of rational thought.

“I am conscious” means that I am aware of the world and can identify things that exist using my mind. I can perceive the things that exist, whether they be cars, animals, or anything else that is presented to my senses. I am actually aware of the world.

Obviously, the fact that we are conscious is forced upon us by every perception or thought we have. If I perceive a lamp, I can see that I am aware of that lamp. If I think about Nazi Germany, I can see that my mind is thinking about Nazi Germany. Every act of my consciousness contains consciousness. Thought and perception are undeniable data of experience.


Having gone through these examples of self evident propositions, we can see that there are in fact self evident propositions, since there can be no doubt about the existence of an objective reality, the law of identity, or consciousness.

Some philosophers have raised objections to the idea of the self evident, and I may address those objections in another post. Keep in mind, though, that even if we do not know the answer to some tricky objection, that casts no doubt on the idea of self evidence or the self evident propositions I have enumerated. We can see, just by looking at them, that any objection to these propositions has to fail, even if we cannot identify the error in a specific objection.

Disjunctive Syllogism

The disjunctive syllogism is a form of deductive reasoning that goes as follows:

  1. Either p is true, or q is true.
  2. p is false.
  3. Therefore, q is true.

This is a deductively valid form of reasoning because, given that p and q are the only alternatives, and p is false, q has to be true. The conclusion follows necessarily from the premises in virtue of the form of the argument.

This form of reasoning is useful when we have a small set of hypotheses, because we can use it to narrow down which one is true. For example, in a murder trial, we might know that the only people at the scene of the crime were Alex and Bob. So, we could reason as follows.

  1. The murderer was either Alex or Bob.
  2. Alex is on video camera in a different room at the time of the murder, so he can’t be the murderer.
  3. Therefore, the murderer was Bob.

Importantly, this requires us to actually have a good reason to believe that Alex and Bob were the only people at the scene of the crime. If there is evidence that there might have been a third person at the scene of the crime, then our argument commits the fallacy of the false dichotomy, because there is a third possibility, namely that the murderer was this third person. This may create “reasonable doubt” in the jury’s mind about whether Bob was the murderer, depending on the other facts.

However, alternative hypotheses cannot be generated arbitrarily. There has to be some reason to take a hypothesis seriously before it becomes an objection to our argument. For example, the defense attorney can’t say “maybe a Martian was the murderer” – that is a doubt, but it is not a reasonable one, and so does not undermine our argument.

Objective Definitions

I was asked on another website how we should treat definitions that aren’t strictly convertible with the concepts that they define. In other words, in mathematics and logic, you can generally just substitute a concept whenever its definition appears, because there is nothing more to the concept than its definition states. You can’t do that with a lot of definitions elsewhere. For example, a horse without hooves is still a horse even though it doesn’t strictly meet the definition of a horse.

However, even when we define a term in mathematics, we don’t generally do so as an end in itself, and if we did our definition would quickly be forgotten or discarded. We formulate our definitions in order to prove things from them. The definition names a useful starting point in our reasoning for seeking out and identifying logical connections that exist objectively.

So, on the view I’m defending, our main goal when we define something isn’t necessarily to come up with a definition in which the subject is strictly convertible with the predicate, it is to identify the essence of the subject. This is a true proposition, in genus and differentia, from which the most of the subject’s known attributes follow, whether causally or logically. If the definition is convertible with the concept it defines, great, but if not, that’s fine too.

So, when we’re defining, say, a horse, the goal shouldn’t be to come up with a proposition that’s true of every horse, although it would be great if we could. We want to find the essence, a proposition which identifies the genus and differentia from which the most of the attributes of horses follow. We want the definition to do this because it will enable us to draw the most true generalizations about horses in our subsequent research, although we may have to take particular exceptions like the occasional horse without hooves into account when we are reasoning about them specifically.