The faculty of reason or intellect must be sharply distinguished from that of sensation. The mind is an active organ with its own internal structure, which it imposes on its contents. Even in perception the mind does not just passively receive contents or ideas from the senses but actively processes, organizes, and structures those contents. Principle of Sufficient Reason. The universe is completely intelligible to reason.
Nothing is arbitrary.
Recall Voltaire's rather unfair spoof of this idea in Dr. Pangloss. However, not all of the so-called empiricists were so pure. Locke, for example, retained a strong rationalist element, as we shall see below. Source of ideas. All ideas, all mental contents, originate in experience.