Montaigne retired and sat down to write and have wise original thoughts.
He realised he had none. He then decided to "assay his character" through writing.
Montaigne published the first "Essays" in 1580.
He spent the rest of his life reflecting through writing. Often arguing with his past self, keeping the past writing in place.
To assay is "to take the measure of".
Montaigne lived a measured life.