Phenomenon and illusion are not the same.  The transcendental distinction between phenomena and things in themselves must not be confused with the distinction common to ordinary life and to physics, in accordance with which we call the rainbow a mere appearance (better, illusion), but the combination of sun and rain which gives rise to this illusion the thing in itself, as that which in universal experience and in all different positions with respect to the senses, is thus and not otherwise determined in intuition, or that which essentially belongs to the intuition of the object, and is valid for every human sensibility (in antithesis to that which only contingently belongs to it, and is valid only for a special position or organization of this or that sense).  Similarly an object always appears to grow smaller as its distance increases, while in itself it is and remains of some fixed size.  And this use of words is perfectly correct, in the physical or empirical sense of “in itself”; but in the transcendental sense the raindrops, also, together with their form and size, are themselves mere phenomena, the “in itself” of which remains entirely unknown to us.  Kant, moreover, does not wish to see the subjectivity of the forms of intuition placed on a level with the subjectivity of sensations or explained by this, though he accepts it as a fact long established.  The sensations of color, of tone, of temperature are, no doubt, like the representation of space in that they belong only to the subjective constitution of the sensibility, and can be attributed to objects only in relation to our senses.  But the great difference between the two is that these sense qualities may be different in different persons (the color of the rose may seem different to each eye), or may fail to harmonize with any human sense; that they are not a priori in the same strict sense as space and time, and consequently afford no knowledge of the objects of possible experience independently of perception; and that they are connected with the phenomenon only as the contingently added effects of a particular organization, while space, as the condition of external objects, necessarily belongs to the phenomenon or intuition of them. It is through space alone that it is possible for things to be external objects for us.  The subjectivity of sensation is individual, while that of space and time is general or universal to mankind; the former is empirical, individually different, and contingent, the latter a priori and necessary.  Space alone, not sensation, is a conditio sine qua non of external perception.  Space and time are the sole a priori elements of the sensibility; all other sensuous concepts, even motion and change, presuppose perception; the movable in space and the succession of properties in an existing thing are empirical data.