How is respiration effected? You ask -- why does air periodically rush intothe lungs? The answer is that influx of air is caused by an enlargement ofthe thoracic cavity, due, partly to depression of the diaphragm, partly tomotion of the ribs. But how can these bony hoops move, and how does motionof them enlarge the cavity? In reply the anatomist explains that though attachedby their ends the ribs can move a little round their points of attachment;he then shows you that the plane of each pair of ribs makes an acute anglewith the spine; that this angle widens when the sternal ends of the ribsare raised; and he makes you realize the consequent dilatation of the cavity,by pointing out how the area of a parallelogram increases as its angles approachto right angles: you understand this special fact when you see it to be aninstance of a general geometrical fact. There still arises, however, thequestion -- why does the air rush into this enlarged cavity? To which comesthe answer that, when the thoracic cavity is enlarged, the contained air,partially relieved from pressure, expands, and so loses some of its resistingPower; that hence it opposes to the pressure of the external air a less pressure;and that as air, like every other fluid, presses equally in all directions,motion must result along any line in which the resistance is less than elsewhere;whence follows an inward current. And this interpretation you recognize asone, when a few facts of like kind, exhibited more plainly in a visible fluidsuch as water, are cited in illustration. Again, after being shown that thelimbs are compound levers acting in essentially the same way as levers ofiron, you would consider yourself as having obtained a partial rationaleof animal movements. The contraction of a muscle, seeming before quite unaccountable,would seem less unaccountable were you shown how, by a galvanic current,a series of soft iron magnets could be made to shorten itself through theattraction of each magnet for its neighbours: -- an alleged analogy whichespecially answers the purpose of our argument, since, whether real or fancied,it equally illustrates the mental illumination that results on finding aclass of cases within which a particular case may perhaps be included. Similarlywhen you learn that animal heat arises from chemical combination, and somay be classed with heat evolved in other chemical combinations -- when youlearn that the absorption of nutrient liquids through the coats of the intestinesis an instance of osmotic action -- when you learn that the changes undergoneby food during digestion, are like changes artificially producible in thelaboratory; you regard yourself as knowing something about the natures ofthese phenomena.
Observe now what we have been doing. We began with special and concretefacts. In explaining each, and afterwards explaining the general facts ofwhich they are instances, we have got down to certain highly general facts:
-- to a geometrical principle, to a simple law of mechanical action, to alaw of fluid equilibrium -- to truths in physics, in chemistry, in thermology.
The particular phenomena with which we set out have been merged in largerand larger groups of phenomena; and as they have been so merged, we havearrived at solutions we consider profound in proportion as this process hasbeen carried far. Still deeper explanations are simply further steps in thesame direction. When, for instance, it is asked why the law of action ofthe lever is what it is, or why fluid equilibrium and fluid motion exhibitthe relations they do, the answer furnished by mathematicians consists inthe disclosure of the principle of virtual velocities -- a principle holdingtrue alike in fluids and solids -- a principle under which the others arecomprehended.
Is this process limited or unlimited? Can we go on for ever explainingclasses of facts by including them in larger classes; or must we eventuallycome to a largest class? The supposition that the process is unlimited, wereany one absurd enough to espouse it, would still imply that an ultimate explanationcould not be reached, since infinite time would be required to reach it.
While the unavoidable conclusion that it is limited, equally implies thatthe deepest fact cannot be understood. For if the successively deeper interpretationsof Nature which constitute advancing knowledge, are merely successive inclusionsof special truths in general truths, and of general truths in truths stillmore general; it follows that the most general truth, not admitting of inclusionin any other, does not admit of interpretation. Of necessity, therefore,explanation must eventually bring us down to the inexplicable. Comprehensionmust become something other than comprehension, before the ultimate factcan be comprehended. §24. The inference which is thus forced on us when we analyze theproduct of thought, as exhibited objectively in scientific generalizations,is equally forced on us by an analysis of the process of thought, as exhibitedsubjectively in consciousness. The demonstration of the relative characterof our knowledge, as deduced from the nature of intelligence, has been broughtto its most definite shape by Sir William Hamilton. I cannot here do betterthan extract from his essay on the "Philosophy of the Unconditioned,"the passage containing the substance of his doctrine.
