(vii) Based upon the quantities and constitutions of objects. (viii) Relation. (ix) Based upon frequency or rarity of occurences. (x) Based upon systems, customs and laws, mythical beliefs, and dogmatic opinions. [1] _Hyp._ I. 36-38. Although Sextus is careful not to dogmatise regarding the arrangement of the Tropes, yet there is in his classification of them a regular gradation, from the arguments based upon differences in animals to those in man, first considering the latter in relation to the physical constitution, and then to circumstances outside of us, and finally the treatment of metaphysical and moral differences. _The First Trope_.[1] That the same mental representations are not found in different animals, may be inferred from their differences in constitution resulting from their different origins, and from the variety in their organs of sense. Sextus takes up the five senses in order, giving illustrations to prove the relative results of the mental representations in all of them, as for example the subjectivity of color[2] and sound.[3] All knowledge of objects through the senses is relative and not absolute. Sextus does not, accordingly, confine the impossibility of certain knowledge to the qualities that Locke regards as secondary, but includes also the primary ones in this statement.[4] The form and shape of objects as they appear to us may be changed by pressure on the eyeball. Furthermore, the character of reflections in mirrors depend entirely on their shape, as the images in concave mirrors are very different from those in convex ones; and so in the same way as the eyes of animals are of different shapes, and supplied with different fluids, the ideas of dogs, fishes, men and grasshoppers must be very different.[5] [1] _Hyp._. I. 40-61. [2] _Hyp._. I. 44-46. [3] _Hyp._. I. 50. [4] _Hyp._. I. 47. [5] _Hyp._. I. 49.