their properties, their elements, their combinations, their proportions, we yet know the simple and general laws, according to which bodies move;) we see clearly, that some of these laws, common to all beings, never contradict themselves; although, on some occasions, they appear to vary, we are frequently competent to discover that the cause becoming complex, from combination with other causes, either impedes or prevents its mode of action being such as in its primitive state we had a right to expect. We know that active, igneous matter, applied to gunpowder, must necessarily cause it to explode: whenever this effect does not follow the combination of the igneous matter with the gunpowder—whenever our senses do not give us evidence of the fact, we are justified in concluding, either that the powder is damp, or that it is united with some other substance that counteracts its explosion. We know that all the actions of man have a tendency to render him happy: whenever, therefore, we see him labouring to injure or destroy himself, it is just to infer that he is moved by some cause opposed to his natural tendency; that he is deceived by some prejudice; that, for want of experience, he is blind to consequences: that he does not see whither his actions will lead him.
If the motion excited in beings was always simple; if their actions did not blend and combine with each other, it would be easy to know, and we should be assured, in the first instance, of the effect a cause would produce. I know that a stone, when descending, ought to describe a perpendicular: I also know, that if it encounters any other body which changes its course, it is obliged to take an
