The first question was the immediate interest of Locke's suc cessor, Berkeley, who declared them both to be upon the same level of reality, both being dependent on the mind, or being ideas, that is mental objects. Berkeley had little difficulty in showing first of all that the primary qualities are as much subject to varia tion with the condition of the observer as the secondary ones; and secondly that the primary ones cannot be experienced apart from the secondary ones—extension must be seen or touched to be ex perienced. If, then, the secondary ones are mental, so also are the primary ones. Berkeley's contention that both sets are on the same level of reality has been generally accepted by all who have been content to regard both of them as sensible objects.
When, as with Kant, a distinction has been readmitted it has been on the ground that extension, for instance, is not a sensible but an a priori form of all external sensibles. Granted, however, that they are both on the same level of reality, the primary qualities may still be the more important. But the significance of Berkeley's result for present philosophy lies in this that it leads directly to the question, are the two sets of qualities alike because both are dependent on mind, or because both are equally independent of mind? Now suppose, as simple realism supposes, that what the mind directly experiences is not its ideas but real objects themselves; that instead of holding its objects to be ideas we hold the so-called ideas to be objects themselves; it is still true that primary and secondary qualities are equally real, this time because both are equally independent of mind.
The general metaphysical difficulties of such an idealism as Berkeley's cannot be fully discussed here. But the idealism which has replaced his still declares that reality is experience and even sentient experience (Bradley) ; and accordingly there is some im patience expressed in either camp when it is asked if secondary qualities are not real. The question still remains whether they are both dependent on mind, or both independent of it. That sec ondaries are still dependent upon us is maintained even by those realists who urge that, at any rate, they depend if not upon our minds at least upon our bodies, that there would not be colours except there were eyes. The answer made by those realists who maintain that secondaries are as much as primaries independent of mind, is that the bodily organs are instrumental to perception, but are not constitutive, even in part, of the perceived. It is one thing to hold that objects cannot be known without a mind to know them, and another to hold that they owe their existence to mind. The doctrine of behaviourism, which has acquired such
strength in American psychology, rests upon the belief that what are called mental acts are nothing but bodily reactions of certain specific sorts to stimulation from objects, and are thus instances of the wider field of organic vital reactions. And some, like the present writer, have expressed the situation for philosophy by saying that in sensation a bodily reaction which is conscious takes place in response to a physical object and that in such action an object or sensum, e.g., a colour, is revealed to the mind. What the exact character of the object in question is may for the moment be deferred.
According to this, secondary and primary qualities alike are independent of mind. An entirely different complexion is put upon the matter in the theory of "neutral monism" advocated by several present philosophers both American (e.g., Holt) and English (e.g., Bertrand Russell). Upon this view the stuff of things is neither material nor mental but neutral; as Bertrand Russell puts it, the mental is more like matter and the material more like mind than is commonly supposed. This doctrine descends from William James who said that physical objects and the ideas of them are but the same thing considered in different relations as, for instance, the same fire may physically destroy a house and mentally suggest the danger of its inhabitants. It is plain that secondary qualities are real on such a view, and that there is no point in asking if they are subjective or objective. Into the grounds of this doctrine there is no space to enter. We may note, however, that it is far closer in its general tendency to idealism as represented by Bradley than it is to any realism which declares that we apprehend objects directly and that we discover what they are in virtue of the specific reactions which the body mind makes towards them. Without examining neutral monism further we may observe that it stands or falls with its initial statement of experience. When we see a patch of red, it is held that the experience is completely described in the words, a red patch occurs. Is this exact, or must we rather say, I am aware, or there is awareness, of a red patch? This question must be left, in order to turn to the second ques tion of the order of priority between primary and secondary qualities. No one has ever suggested that the two sets of qualities are upon the same level in this respect. But it is noticeable that in the more recent English philosophy the old priority of the primaries seems to give way to the priority of the secondaries.