We can now, therefore, perceive what is the generic distinction between sciences which can be made Deductive, and those which must as yet remain Experimental. The difference consists in our having been able, or not yet able, to discover marks of marks. If by our various inductions we have been able to proceed no further than to such propositions as these, _a_ a mark of _b_, or _a_ and _b_ marks of one another, _c_ a mark of _d_, or _c_ and _d_ marks of one another, without anything to connect _a_ or _b_ with _c_ or _d_; we have a science of detached and mutually independent generalizations, such as these, that acids redden vegetable blues, and that alkalies colour them green; from neither of which propositions could we, directly or indirectly, infer the other: and a science, so far as it is composed of such propositions, is purely experimental. Chemistry, in the present state of our knowledge, has not yet thrown off this character. There are other sciences, however, of which the propositions are of this kind: _a_ a mark of _b_, _b_ a mark of _c_, _c_ of _d_, _d_ of _e_, &c. In these sciences we can mount the ladder from _a_ to _e_ by a process of ratiocination; we can conclude that _a_ is a mark of _e_, and that every object which has the mark _a_ has the property _e_, although, perhaps, we never were able to observe _a_ and _e_ together, and although even _d_, our only direct mark of _e_, may not be perceptible in those objects, but only inferrible. Or, varying the first metaphor, we may be said to get from _a_ to _e_ underground: the marks _b_, _c_, _d_, which indicate the route, must all be possessed somewhere by the objects concerning which we are inquiring; but they are below the surface: _a_ is the only mark that is visible, and by it we are able to trace in succession all the rest.