1. _Observation_ of facts--including under this head that _artificial observation_ which is called _experiment_. 2. That process of tying up similar facts into bundles, ticketed and ready for use, which is called _Comparison_ and _Classification_,--the results of the process, the ticketed bundles, being named _General propositions_. 3. _Deduction_, which takes us from the general proposition to facts again--teaches us, if I may so say, to anticipate from the ticket what is inside the bundle. And finally-- 4. _Verification_, which is the process of ascertaining whether, in point of fact, our anticipation is a correct one. Such are the methods of all science whatsoever; but perhaps you will permit me to give you an illustration of their employment in the science of Life; and I will take as a special case the establishment of the doctrine of the _Circulation of the Blood_. In this case, _simple observation_ yields us a knowledge of the existence of the blood from some accidental haemorrhage, we will say; we may even grant that it informs us of the localisation of this blood in particular vessels, the heart, &c., from some accidental cut or the like. It teaches also the existence of a pulse in various parts of the body, and acquaints us with the structure of the heart and vessels. Here, however, _simple observation_ stops, and we must have recourse to _experiment_. You tie a vein, and you find that the blood accumulates on the side of the ligature opposite the heart. You tie an artery, and you find that the blood accumulates on the side near the heart. Open the chest, and you see the heart contracting with great force. Make openings into its principal cavities, and you will find that all the blood flows out, and no more pressure is exerted on either side of the arterial or venous ligature. Now all these facts, taken together, constitute the evidence that the blood is propelled by the heart through the arteries, and returns by the veins--that, in short, the blood circulates. Suppose our experiments and observations have been made on horses, then we group and ticket them into a general proposition, thus:--_all horses have a circulation of their blood_. Henceforward a horse is a sort of indication or label, telling us where we shall find a peculiar series of phaenomena called the circulation of the blood. Here is our _general proposition_, then. How, and when, are we justified in making our next step--a _deduction_ from it? Suppose our physiologist, whose experience is limited to horses, meets with a zebra for the first time,--will he suppose that this generalisation holds good for zebras also? That depends very much on his turn of mind. But we will suppose him to be a bold man. He will say, "The zebra is certainly not a horse, but it is very like one,--so like, that it must be the 'ticket' or mark of a blood-circulation also; and, I conclude that the zebra has a circulation."