The idea that there are certain "laws" of learning (similarity, contrast, contiguity) can be traced to Aristotle. He maintained that external stimuli cause small movements in the vessels to the dominant heart, the vestiges of which can be linked to one another. Aristotle's laws of learning were incorporated into the writings of Hobbes, Locke, and Berkeley, men who said nothing about the physiological correlates of mental associations. This left the door open for David Hartley to combine mental associationism with the Newtonian idea that sensations can cause minute particle vibrations in the nerves. Hartley's amalgam of psychology, philosophy, and neurology was first presented in 1746, as a "trial balloon" at the end a little-known monograph on a treatment for kidney stones. It was repeated three years later in his better-known Observations on Man. In many ways, modern psychobiological connectionism can be traced back to Hartley's Conjectures of 1746, in which Aristotle's original thoughts were modified with then current ideas about functions of the mind and the nervous system.