Sensations instinctively lead us to believe in the existence of an external world; and if this belief be submitted to the examination of reason, it confirms the same truth, resting upon the general ideas of cause and effect. The pure understanding knows certain principles, and assents to them as to necessary truths. The senses, if these principles be submitted to their experience, confirm them as much as their own perfection, or that of the instruments which they use, permit. "All the radii of a circle are equal." This is a necessary truth. The senses perceive no perfect circle, but they see that the more perfect the instrument is with which they construct it, so much more nearly equal are the radii. "There is no change without a cause to produce it." The senses cannot prove the proposition in all its universality, for they are by nature limited to a determinate number of particular cases; but, so far as their experience goes, they discover the order of such a dependence in the succession of phenomena. The senses mutually aid each other. The sensation of one sense is compared with those of the others, when there arises a doubt as to its correspondence with its object. We seem to hear the whistling of the wind; but our hearing has more than once deceived us: to make sure of the truth we look at trees or other objects. We see a figure, but there is not light sufficient to distinguish it from a shadow, we approach and touch it. The intellectual and moral faculties also exercise a salutary influence upon each other. Ideas rectify sentiments, and sentiments ideas. The value of the ideas of one order is verified by those of another order; and the same with sentiments. Pity for one suffering punishment inspires the pardon of all criminals; the indignation inspired at seeing the victims of crime, induces to the application of punishment; both sentiments involve something that is good; but the one would engender impunity, the other cruelty; it is to temper them that the ideas of justice exist. But this justice in its turn might pronounce excessively absolute sentences; justice is one, and the circumstances of peoples very different. Justice only considers the culpability, and consequently pronounces such sentences. A sentence may not be proper; for here come in other moral ideas of a distinct order, the amendment of the guilty joined with reparation made to the injured party; ideas also of public convenience which are not repugnant to sound morals, and which may direct their application.