- (Premise) Every lie is an assertion.
- (Premise) Every assertion is a speech act.
- (Premise) Some gestures are lies.
- Therefore, some gestures are assertions.
- Therefore, some gestures are speech acts.
The conclusion of this argument should be obvious, but apparently it is controversial.
Evidence for 1: That lies are assertions is the least common denominator of standard accounts of lying ("A lie is an assertion of something one disbelieves", "A lie is an assertion of something one believes to be false", "A lie is an assertion of something one believes to be false and intends to deceive by means of", etc.)
Claim 2 seems obvious: assertion is a paradigm of speech act, the speech act most studied.
Claim 3 is of course the center of the argument. But it's easy to see. Sam swears to tell the truth, the whole truth and nothing but the truth. He is asked if the man he saw with the knife is in the court. He says "Yes." He is asked to point out the man. He deliberately points to the defendant while fully aware that the man he saw with the knife was the prosecutor, because the prosecutor paid him off. Clearly, Sam is lying. But he is doing so by pointing, and pointing (in our culture) is a gesture.
Moreover, notice that Sam's action violates his oath to tell the truth, the whole truth and nothing but the truth, which also shows that his pointing is a kind of telling.