The third sense is smell, which is caused by the nostrils transmitting the vapours to the brain: and it is bounded by the extreme limits of the anterior ventricle of the brain. It is the faculty by which vapours are perceived and apprehended. Now, the most generic distinction between vapours is whether they have a good or an evil odour, or form an intermediate class with neither a good nor an evil odour. A good odour is produced by the thorough digestion in the body of the humours. When they are only moderately digested the intermediate class is formed, and when the digestion is very imperfect or utterly wanting, an evil odour results.