Why Montaigne deserves founder status
Francis Bacon, author of the first published book of English essays, also put novelty and tradition in uneasy relation in the dedication of a manuscript version of his essays, claiming that ‘the word is late, but the thing is ancient’; in other words, that while calling a composition an ‘essay’ was new, the form of writing it designated was not. Like Montaigne’s idea of the human person, the essay is at once idiosyncratically novel, and a reworking of inherited features.
While I don't think Montaigne's idiosyncracies should be mistaken for the entire genre—as it seems this book does frequently—I do think he deserves the title of its founder. He uniquely possessed a constellation of founding traits.
There were definitely proto-essayists from Asia, centuries before, and you could even trace the genre back to Plutarch and Seneca, each of who wrote 100+ essays covering a similar range of miscellaneous topics. The free-wheeling portfolio, covering seemingly any topic, is definitely a trait of an essayist. But that's only one of several sensibilities, all of which haven't been combined until combined.
The two other things Montaigne brought to the essay were:
(1) An evolving morality. The ancients would go through essayistic digressions, but ultimately landed at predictable lessons that congealed with their school of thought. They were predictable. Each essay was a node in a cohesive moral philosophy. Montaigne, on the other hand, was a shapeshifter, always willing to betray his older self, making him full on contradictions. He was the first mind in motion, captured on the page in near totality, reaching for unknown territory.
(2) A meta impulse. Montaigne was, more so than his predecessors, write about what he was doing, expand the conception of what was possible, and give it a name. Contrary to what Bacon says here, naming it is not irrelevant. The name creates a handle for others to emulate. And what was emulated was something central to Bacon; even though it was he who is given credit for the beginnings of the scientific, that all stems from a process that is uniquely Montaignean: to suspend all conclusions, and empirically trust your own experiences to make sense of the data and drive a new perspective forward. Montaigne didn't experiment with the natural world, but he intuitively followed a quasi-scientific method for his own self.