His thought was that by attribute we mean that which is not independent, but must be referred to something else; by substance, we mean that which exists independently and is not referred to any other thing.  It seemed to follow that there could be only one substance.

Spinoza modified Descartes’ doctrine in that he refused to regard mind and matter as substances at all.  He made them unequivocally attributes of the one and only substance, which he called God.

The thought which influenced Spinoza had impressed many minds before his time, and it has influenced many since.  One need not follow him in naming the unitary something to which mind and matter are referred substance.  One may call it Being, or Reality, or the Unknowable, or Energy, or the Absolute, or, perhaps, still something else.  The doctrine has taken many forms, but he who reads with discrimination will see that the various forms have much in common.

They agree in maintaining that matter and mind, as they are revealed in our experience, are not to be regarded as, in the last analysis, two distinct kinds of thing.  They are, rather, modes or manifestations of one and the same thing, and this is not to be confounded with either.

Those who incline to this doctrine take issue with the materialist, who assimilates mental phenomena to physical; and they oppose the idealist, who assimilates physical phenomena to mental, and calls material things “ideas.”  We have no right, they argue, to call that of which ideas and things are manifestations either mind or matter.  It is to be distinguished from both.

To this doctrine the title of Monism is often appropriated.  In this chapter I have used the term in a broader sense, for both the materialist and the spiritualist maintain that there is in the universe but one kind of thing.  Nevertheless, when we hear a man called a monist without qualification, we may, perhaps, be justified in assuming, in the absence of further information, that he holds to some one of the forms of doctrine indicated above.  There may be no logical justification for thus narrowing the use of the term, but logical justification goes for little in such matters.

Various considerations have moved men to become monists in this sense of the word.  Some have been influenced by the assumption—­one which men felt impelled to make early in the history of speculative thought—­that the whole universe must be the expression of some unitary principle.  A rather different argument is well illustrated in the writings of Professor Hoeffding, a learned and acute writer of our own time.  It has influenced so many that it is worth while to delay upon it.

Professor Hoeffding holds that mental phenomena and physical phenomena must be regarded as parallel (see Chapter IX), and that we must not conceive of ideas and material things as interacting.  He writes:[1]—­