This law of harmony has been recognized and to some extent defined by early philosophers and writers as well as by those of recent date. [SN: _Plato_] It was recognized and referred to by Plato, who said that the world offers the material in graceful and beautiful forms; or again that there is no difficulty in seeing that grace or the absence of grace is an effect of good or bad rhythm ... that beauty of style and harmony and grace and good rhythm depend on simplicity. He also refers to art as representing proportion, harmony, or unity among the parts. His thought is that there is an absolute principle of beauty which reveals itself in natural objects. [SN: _Aristotle_] Aristotle expressed the opinion that the essential qualities of beauty are order and symmetry. [SN: _Knight_] Knight refers to the appreciation of symmetry and proportion on the part of the Greek people and he concludes that the knowledge of this same law of symmetry and its appreciation was doubtless the basis of Greek art. [SN: _Kant_] Kant in his philosophy refers to this same law of symmetry, grace, and beauty in nature. He says: “The beautiful forms displayed in the organic world all plead eloquently on the side of the realism of the aesthetic finality of nature in support of the plausible assumption that beneath the production of the beautiful there must lie a preconceived idea in the producing cause--that is to say, an end acting in the interest of our imagination. Flowers, blossoms, even the shapes of plants as a whole, the elegance of animal formations of all kinds, unnecessary for the discharge of any function on their part, but chosen as it were with an eye to our taste; and, beyond all else, the variety and harmony in the array of colours (in the pheasant, in crustacea, in insects, down even to the meanest flowers) so pleasing and charming to the eye, but which, inasmuch as they touch the bare surface and do not even here in any way affect the structure of these creatures--a matter which might have a necessary bearing on their internal ends--seem to be planned entirely with a view to outward appearance: all these lend great weight to the mode of explanation which assumes actual ends of nature in favor of our aesthetic judgment.” [SN: _Blackie_] John Stuart Blackie refers to qualities in nature which create spontaneously in the mind a degree of pleasure because of their symmetry and beauty. He says: “There must be, therefore, in nature and in the constitution of things certain qualities which, being superinduced upon the useful, or mere fitness to achieve a practical end, create in the mind the pleasant sensations which arise spontaneously on the perception of a beautiful object.”