QuestionsCan we be certain of anything?"If I am deceived, I am"
Can we be certain of anything?Yes, through reason

"If I am deceived, I am"

Augustine of Hippo 354 · 430Believe to understand: God in the innermost self.
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What it claims

I am, and on this point I cannot be deceived: the sceptic cannot make me doubt it.

Why

To be deceived, one must exist: even if I am mistaken, I am.

In short

""If I am deceived, I am"" is an argument by Augustine of Hippo (City of God, XI, 26) on the question "Can we be certain of anything?" (answer: yes, through reason). Its claim: I am, and on this point I cannot be deceived: the sceptic cannot make me doubt it. According to AskPhilosophy's logic check (Z3 solver), the argument is valid only if one grants a premise the author leaves unstated (P3). AskPhilosophy presents 3 objections, 1 of them with a documented reply, and the original text behind each premise, checked word for word.

City of God, XI, 26
Valid with an implicit premise

The sceptics of the Academy, Plato's heirs, held that nothing can be known for certain: "what if you are deceived?". Augustine answers them, twelve centuries before Descartes' cogito, that doubt itself proves something. He already says so around 388-395 in On Free Choice (II, 3, 7), then in its sharpest form in the City of God.

P1One who is not cannot be deceived: to be deceived, one must exist.
P2I believe that I am, that I know, and that I love being and knowing.
P3 · implicit premiseThere are only two cases: either I am deceived or I am not; and if I am not deceived in believing that I am, then I am.
Therefore I am, and on this point I cannot be deceived: the sceptic cannot make me doubt it.

click a premise to reject it

If you accept every premise, logic forces you to accept the conclusion. So the debate is about the premises.
Checked by a logic solver: the premises cannot all be true while the conclusion is false.
Without the implicit premise: Without P3, the computer finds a strange case: I believe I am, I am not deceived, and yet I do not exist. Absurd, but it has to be said for the logic to be complete.
Valid with P3, which only says that a belief which is not an error is true. Augustine finds it too obvious to state. The argument's strength lies elsewhere: even in the worst case (I am deceived), the conclusion holds. As with Descartes, it is not really a multi-step inference but something self-evident. The difference is in the use: Augustine uses it to turn inward and rise towards God, Descartes to ground all of science.
Analogy. If someone tells you "maybe you are dreaming everything", you can doubt the room, the people, your own body. But to be fooled by a dream, someone must be dreaming. Error itself assumes you are there.

Objections

3

Each objection is a conversation: click to see who replies and where the debate stands.

The sceptics of the Academy (Carneades, Cicero) attacks premise 2

Our senses, dreams and imagination constantly deceive us. How can you be sure of anything, even of what you think you know about yourself?

Augustin replies
Augustin
Here I go through no bodily sense and no image: what I grasp is not an outside object, but the very fact that I am, that I know and that I love. No illusion of the imagination can slip in.
Nietzsche 1886 · attacks premise 1

"Someone must be there to be deceived" comes from grammar, not from facts: we believe every activity has an agent. There is error, there is thinking; that there is an "I" behind it is an assumption, not a certainty.

no reply
No reply possible: objection raised in 1886, after the author's death (430).
Hume 1739 · attacks the conclusion

Grant "I am". But who is this "I"? When I look within, I only ever find a perception (heat, cold, pleasure, pain), never a self that has them. Augustine's certainty does not say what I am.

no reply
No reply possible: objection raised in 1739, after the author's death (430).