The last and sixth subject of knowledge is the intimate relation of objects through their similarities and differences. These relations are different in different cases. For instance, the objects of the senses are different from the result attained by means of them. An object may be united with one thing for one purpose and with another for another; and an effect may be the effect of something different from what we believe it to be. Similar considerations arise in respect of what constitutes a valid argument.
The objects of the senses: The objects of the senses are well known; but when a person attains to them, the result achieved is different from the objects themselves. An object which is closely connected or united with another, may be associated with it for one purpose, and opposed to it in another; and even so an effect may be the effect of something other than we believe it to be.
That is due to the fact that the result is different from the object that produces it.
Conditions of a valid argument: That which is not proved, cannot be advanced as a valid argument; nor that which does not exist, or is doubtful. An argument bearing on what relates to the connection of the objects of the senses with the soul is not invalid; but it can have its limitations.
We can understand the character and function of the mind in the same manner, for it acts in association with the senses and the soul. Similarly, we can know that the soul exists, because of the functions of the body, senses, and the mind, and the experience of pleasure and pain, and also the character of the "I" within us. All souls are alike so far as the experience of pleasure and pain, and the acquisition of knowledge are concerned but otherwise the souls are really many, and different from one another.
Mind: We know that there is a mind when the soul comes into contact with the senses, and when there is knowledge or the absence of it. Mind is a substance like Air, and eternal (or non-eternal) like it. A creature has only one mind; and we say so because there is no simultaneity of knowledge or action.
Soul: Breathing in, breathing out, closing and opening the eyes, the manner of living, the course of the mind, the changes within the senses, pleasure, pain, desire, aversion, and efforts, all these show that there is a soul. It is a substance, and eternal (or non-eternal) like Air.
Proof of the soul's existence: When we are in physical contact with a person, we can say that he is so and so; and it is for this reason that pratyaksha or sense-perception is necessary for the purpose of acquiring scientific knowledge. But so far as the "I" within us is concerned, since it is different from everything else that we see sense-perception cannot be a proper means of acquiring its knowledge.
But we find that when we say "I", it refers to the soul and soul alone; and that is as good a proof of the existence of the soul as pratyaksha or sense-perception can provide; and so this knowledge of the soul may be said to be derived by means of a pratyaksha of a different kind. When, however, we say from the peculiar manner of his going, that it is a particular person, what is perceptible is his ahankara or the I-as-an-actor, so far as his body is concerned.
Similarity and difference between souls: We find that the experience of pleasure, pain, and knowledge is common to all; and so we might say that, to this extent, the souls of all creatures are the same. But the souls are really many, and each is different according to the state in which it is born.
The attributes of substances: Let us now consider the idea of attributes of substances. Certain things are said to be eternal, while others non-eternal; and we need to understand that the eternal is that which exists; and that it is an effect, but an effect without a cause.
An attribute is something that is abstract, and we can understand its idea from what is concrete. For instance, we can understand the idea of largeness from a large number of substances and forms; and we can get the idea of form from a number of things having a form. It is in this manner that we can understand the idea of attributes and character of things.
The Eternal: The eternal is that which exists, but has no cause. Its proof is that it is an effect, but an effect without a cause. This is an exception to the common rule that there can be no effect without a cause, for the rule applies to things that are non-eternal, and not eternal.
From concrete to abstract: We can understand the idea of largeness when we have a large number of substances and forms. Similarly, we can have an idea of form, when we collect together a large number of substances with different forms; and we can form an idea of taste, smell, and touch in the same manner. It is in this way that we have a perception of names, measures, individual existence of objects, union and separation, priority and posteriority, and action, indeed all knowledge relating to the attributes and a character of things.
Characteristics of substances: All substances have three characteristics, body, sense-organs, and sphere of activity. All of them do not consist of the five "elements"; but they are composed of atoms, which combine in them in different ways. We may classify the bodies of creatures in two ways, those born of the womb, and those not born of the womb.
All substances made of the Earth and other "elements" have three characteristics, body, sense-organs, and sphere of activity. There is no ocular evidence of the combinations of all substances; and all of them do not consist of the five "elements", for some consist of only three. This, however, does not mean that we deny that the atoms of which they are composed can combine in different ways.
Two kinds of creatures: The bodies of creatures may be said to be of two kinds, those born of the womb, and those not born of the womb. This is a universal rule of division, and is not restricted by country or place.