Around 266 BC, Arcesilaus became head of the Academy. This phase, known as the Middle Academy, strongly emphasized philosophical skepticism. It was characterized by its attacks on the Stoics and their assertion of the certainty of truth and our knowledge of it. The New Academy began with Carneades in 155 BC, the fourth head in succession from Arcesilaus. It was still largely skeptical, denying the possibility of knowing an absolute truth; both Arcesilaus and Carneades argued that they were maintaining a genuine tenet of Plato.