3. In this way the distinction of primary and secondary qualities is definite and precise. But when men attempt to draw the distinction by guess, without any scientific principle, the separation of the two classes is vague and various. I have, in the _History of Scientific Ideas_[303], pointed out some of the variations which are to be found on this subject in the writings of philosophers. Sir William Hamilton[304] has given an account of many more which he has compared and analysed with great acuteness. He has shown how this distinction is treated, among others, by the ancient atomists, Leucippus and Democritus, by Aristotle, Galen, Galileo, Descartes, Boyle, Malebranche, Locke, Reid, Stewart, Royer-Collard. He then proceeds to give his own view; which is, that we may most properly divide the qualities of bodies into _three_ classes, which he calls _Primary_, _Secundo-primary_, and _Secondary_. The former he enumerates as 1, Extension; 2, Divisibility; 3, Size; 4, Density or Rarity; 5, Figure; 6, Incompressibility absolute; 7, Mobility; 8, Situation. The Secundo-primary are Gravity, Cohesion, Inertia, Repulsion. The Secondary are those commonly so called, Colour, Sound, Flavour, Savour, and Tactical Sensation; to which he says may be added the muscular and cutaneous sensation which accompany the perception of the Secundo-primary qualities. "Such, though less directly the result of foreign causes, are Titillation, Sneezing, Horripilation, Shuddering, the feeling of what is called Setting-the-teeth-on-edge, &c." The Secundo-primary qualities Sir William Hamilton traces in further detail. He explains that with reference to Gravity, bodies are _heavy_ or _light_. With reference to Cohesion, there are many coordinate pairs, of which he enumerates these:--_hard_ and _soft_; _firm_ and _fluid_,--the fluid being subdivided into _thick_ and _thin_; _viscid_ and _friable_; _tough_ and _brittle_; _rigid_ and _flexible_; _fissile_ and _infissile_; _ductile_ and _inductile_; _retractile_ and _irretractile_; _rough_ and _smooth_; _slippery_ and _tenacious_. With reference to Repulsion he gives these qualities:--_compressible_ and _incompressible_; _elastic_ and _inelastic_. And with reference to Inertia he mentions only _moveable_ and _immoveable_. I do not see what advantage is gained to philosophy by such an enumeration of qualities as this, which, after all, does not pretend to completeness; nor do I see anything either precise or fundamental in such distinctions as that of elasticity, a mode of cohesion, and elasticity, a mode of repulsion. But a question in which our philosophy is really concerned is how far any of these qualities are _universal_ qualities of matter. Sir W. Hamilton holds that they are none of them necessary qualities of matter, and therefore of course not universal, and argues this point at some length. With regard to one of his Secundo-primary qualities, I will make some remarks.