News from the Vault: Proving the Negative, or, Why That Invisible Pink Unicorn Isn't
Propositions assert some argument is true, such as 2 + 2 ≠ 4. Such an argument must be both valid and sound. The validity of it, a question of knowledge, must first be evaluated. The soundness of it, a question of the actual existence of the subject, is evaluated secondarily. The first can be determined by inverting the statement, and if the statement is self-refuting, the statement must be true, and if not, it cannot be sound, and may be invalid as well. For a claim to be sound, it must be valid and its premises true.
The inversion here is 2 + 2 = 4, which is not self-refuting and thus disconfirms the statement as unsound ( it's premises are false ) and invalid as 2 + 2 = 4 by definition.
It is the case that there exists an invisible pink unicorn, where invisible means light passes through the object completely, and pink means all frequencies of color are absorbed except pink, which is reflected back to the view.