Hence it follows that the leading doctrine laid down by Lord Bacon as applicable to the investigations of Physical science applies equally in this instance to the researches of the Philologist; I allude to the following fundamental maxim: Experience is the only legitimate guide to _Truth_; hence an accurate investigation of those facts which are _within_ the limits of our historical knowledge, forms the only admissible basis of deduction, with respect to those facts which are _beyond_ the range of our actual experience. 2. Not less applicable is the same maxim in elucidation of the second proposition, viz.: “That the differences which exist between individual languages may be explained, consistently with the proofs of original unity, by causes still in operation.” This principle may be applied in the following manner: There are certain languages of which the original unity can be proved, either by the extrinsic evidence of history, or by the gradual approximation they display as we ascend from modern to earlier epochs, and compare modern with _ancient_ specimens. We can show, by means of the like evidence, the progressive changes they have undergone, and the nature of the existing differences which have been the result of those changes. There is another class of languages which came into existence during periods with regard to which we do _not_ possess the light of history; and the only source from which we can draw our conclusions, with respect to the relations that originally existed between them, is the internal evidence afforded by the composition and structure of those languages themselves. History being silent, this is the only clue by which we can determine whether they were originally distinct, or derived from a common source. But by what rules are we to be guided in the deductions we may form from the mere texture of dialects of the second class? The answer is, that the rules to be pursued in forming our conclusions, with respect to the original relations of those languages which _can not_ be historically traced to their source, must be drawn from the experience furnished by that class of languages of which the transitions _can_ be traced by means of the independent evidence of history. It will be shown that the _existing_ relations between these two different classes of languages _are_, and therefore we may infer that the _original_ relations _were_, the same. By the adoption of these principles of investigation as regards both: 1, The Resemblances, and also 2, The Differences, which Human Tongues display, the great maxim of Lord Bacon’s philosophy will become legitimately applicable to language, and the researches of the Philologist may be directed by the same criteria, and his conclusions vindicated by the same tests as those which apply to the investigations of the inquirer into Physical phenomena.