Lesson 3 · about 50 minutes
How We Know
How knowledge moves from the senses to the intellect, what truth is, and why scepticism and relativism undermine themselves.
By the end of this lesson you will be able to
- Trace how knowledge begins in the senses and is completed by the intellect
- Define truth as the agreement of the mind with reality
- Answer the sceptic and the relativist using first principles such as non-contradiction
Read first
- Epistemology — Can we know truth? The certitude of knowledge and the refutation of scepticism.
Start here: can you trust your eyes?
Look at a straight stick standing half in a glass of water. It looks bent. Your eyes have fooled you.
So can you ever trust your senses? And if you can’t trust your senses, can you know anything at all? These questions are the heart of epistemology, the part of philosophy that asks how we know.
Knowledge begins in the senses
Aquinas teaches that all our knowledge starts with the senses. A baby learns what a dog is by seeing dogs, hearing them bark and patting their fur. Nothing reaches the mind except by first passing in some way through the senses.
But knowledge doesn’t stop there. The senses grasp particular things: this brown dog, barking now. The intellect grasps what things are: the nature “dog”, which is true of every dog that ever lived. You have never seen “dog in general”, yet you understand it perfectly. That is the intellect at work.
| Power | Grasps | Example |
|---|---|---|
| Senses | Particular things, here and now | This dog, this colour, this sound |
| Intellect | Natures and universal truths | What a dog is; “every whole is greater than its part” |
What about mistakes?
The bent stick shows the senses can err. But notice how you discovered the error: by looking more carefully, touching the stick, and thinking about how water bends light. You corrected the senses using the senses and reason together.
That’s the key point. Errors are fixed by better knowing, not by giving up on knowing. If we could know nothing, we could never discover that we had been wrong.
What truth is
Aquinas gives a simple definition, borrowed from earlier thinkers: truth is the agreement of the mind with reality (in Latin, adaequatio rei et intellectus).
If I say “it is raining” and it is raining, my statement is true. If it isn’t raining, my statement is false, however sincerely I believe it, however many people agree with me, and however much I would like it to be true.
Truth depends on how things are, not on how we feel about them.
Two challenges, and the answers
The sceptic says: “We can’t know anything for certain.” But is the sceptic certain of that? If so, he knows at least one thing, and his claim is false. If not, why should anyone accept it?
The relativist says: “That’s true for you, but not for me.” This confuses believing with being true. Two people can believe opposite things about the same fact, but they cannot both be right. The same thing cannot both be and not be at the same time and in the same respect. Aristotle calls this the principle of non-contradiction, and he argues that no one can genuinely deny it, because every attempt to argue against it has to use it.
The relativist’s own claim also defeats itself: “all truth is relative” is offered as true for everyone.
Read the texts
Read Summa Theologica I, q.16, a.1 and Aristotle, Metaphysics IV.3. As you read, ask:
- Why does Aquinas say truth is found first in the intellect rather than in things?
- Why does Aristotle think the principle of non-contradiction is the most certain of all principles?
Primary texts
- St Thomas Aquinas, Summa Theologica I, q.16, a.1 Truth is found in the intellect insofar as it is conformed to the thing understood.
- Aristotle, Metaphysics IV.3 The principle of non-contradiction - the same thing cannot both be and not be in the same respect at the same time.
Check your understanding
Choose an answer for each question, then check your score.
Think and discuss
Talk these over with your class or family, or write a short answer to each.
- How did you discover the illusion was an illusion? What does that show about our senses?
- If "all truth is relative," is that claim relative too?
- Is "true for me" the same as "I believe it"? What is the difference?
- Can two people hold contradictory beliefs about the same fact and both be right?
- What would happen to science, courts and friendships if nobody could know the truth?
Key takeaway
"That's just your truth" doesn't hold up.
Homework: Write a one-page reflection: "Something I know for certain, and how I know it."