Within the scope of these three Sciences, therefore, our information is clearly defined, positive, and indisputable. The conclusions to which we are led by their Principles can no more be gainsayed than human existence can be doubted. While time shall last, while mankind shall endure, while the human Mind is constructed on its present basis; while, in fine, there is a possibility for the exercise of Thought in any way conceivable to the existing Mentality of the universe, the Laws of Mathematics, of Astronomy, and of Physics can be apprehended in no way different from that in which they are now apprehended. There is _no conceivable possibility_ that subsequent investigations will show them to be erroneous or defective. They stand upon a foundation of Proof as unalterable as the fiat of Fate or the decrees of the Almighty, which can neither be shaken nor destroyed. It is between these three Mathematical Sciences, on the one side, and all other domains of intellectual investigation on the other, that a line of distinct demarcation must be drawn, in any Classification of our so-called Knowledge, in accordance with any method of classification known to the scientific world at large. Not that the Laws or Principles which lie at the base of all other departments of the universe are not as stable, as definite, and as infallible as those which inhere in the Sciences which have been specially indicated. But that, as yet, the endeavor to apprehend fundamental Principles, in other spheres than these, has been attended with only partial success; and hence, the ability to establish a Mathematical or Demonstrable basis for other regions of Thought is yet wanting, so far as is commonly known. When, therefore, we emerge from the domains of Mathematics, Astronomy, and Physics, we are leaving the field of _positive assurance_, of _undeniable_ truth, and entering the realms where opinion, conjecture, and variable degrees of certainty prevail. _The Facts of Observation may be, indeed, as plain here as elsewhere and as firmly established. But the conclusions drawn from them, the Scientific Principles assumed to be established, may be erroneous or defective, and the power of prevision, the great test of Scientific accuracy, is proportionally wanting._ Derived, as we have hitherto seen these conclusions to be, from Phenomena, on the supposition that a given range of Observation will secure all the essential Principles which appertain to the _whole_ of the Phenomena included in the range, we can never be _entirely sure_ that our basis of Facts is sufficient for our purpose, and hence the _possibility_ of error always exists.