Source: p 132, Philosophy: A Complete Introduction (2012) by Prof. Sharon Kaye MA PhD in Philosophy (U. Toronto)
But how is lying self-contradictory?
[1.] Kant suggests that the very act of speaking presupposes honesty. [2.] That is, why would you make verbal sounds for me to hear unless there was an underlying assumption between us that you were trying to communicate something? When you lie you're not communicating but undermining communication. So your very act negates itself ─ a self-contradiction.
I know that Kant was a deontologist, but appearing to presuppose unreasonably too much, 1 does not convince me. Here is an answer to 2 that rebuts 1: Suppose that most honest people's speaking does evidence the assumption in 2 (that the honest people try to communicate something); but unfortunately, not having studied Logic at university, they may unintentionally commit Logical Fallacies. Then these honest people did not speak honestly, but they did INTEND to speak honestly?