For hundreds of years, people who were unable to write attested their consent by scratching an “X” on documents they were unable to read. Why X? Why not A, or I, or O?
Of course, there’s the thought of the cross, which in Christian countries might stand as the sign of any individual.
I’ve long thought— perhaps whimsically— that the validity of “X” as a marker of intention comes from its intersection of two opposite lines. Anything, even a branch falling from a tree, can scratch a diagonal line on a surface, but to do the same thing in the opposite direction and have the two lines meet at a point bespeaks awareness and intent, which a judicially recognized signature aims to confirm. The second line of the “X” is supposed to be a minimal extra added on to nature, and once you’ve done that, you’ve started to act in the world, to “persist and sign” as they say in French.