18. All that is here said is highly philosophical and valuable; but we may observe that the investigation of _specific forms_ in the sense in which some writers had used the phrase, was by no means a frivolous or unmeaning object of inquiry. Bacon and others had used _form_ as equivalent to _law_[224]. If we could ascertain that arrangement of the particles of a crystal from which its external crystalline form and other properties arise, this arrangement would be the _internal form_ of the crystal. If the undulatory theory be true, the _form_ of light is transverse vibrations: if the emission theory be maintained, the _form_ of light is particles moving in straight lines, and deflected by various forces. Both the terms, _form_ and _law_, imply an ideal connexion of sensible phenomena; form supposes matter which is moulded to the form; law supposes objects which are governed by the law. The former term refers more precisely to existences, the latter to occurrences. The latter term is now the more familiar, and is, perhaps, the better metaphor: but the former also contains the essential antithesis which belongs to the subject, and might be used in expressing the same conclusions. But occult causes, employed in the way in which Newton describes, had certainly been very prejudicial to the progress of knowledge, by stopping inquiry with a mere word. The absurdity of such pretended explanations had not escaped ridicule. The pretended physician in the comedy gives an example of an occult cause or virtue. Mihi demandatur A doctissimo Doctore _Quare_ Opium facit dormire: Et ego respondeo, _Quia_ est in eo _Virtus dormitiva_, Cujus natura est sensus assoupire. 19. But the most valuable part of the view presented to us in the quotation just given from Newton is the distinct separation, already noticed as peculiarly brought into prominence by him, of the determination of the _laws_ of phenomena, and the investigation of their _causes_. The maxim, that the former inquiry must precede the latter, and that if the general laws of facts be discovered, the result is highly valuable, although the causes remain unknown, is extremely important; and had not, I think, ever been so strongly and clearly stated, till Newton both repeatedly promulgated the precept, and added to it the weight of the most striking examples.