Substance in modern philosophy

One central issue of modern philosophy was the nature of “substance.” The real world, they thought, had to consist of substances, of which the appearances of things are qualities. We can see some of the problem by considering the following sentences:

x is a lamb
x has floppy ears
x is brown
x weighs 12 pounds
x is running to the school
x is Mary’s pet

It seemed that there had to be something corresponding to the “x” here. “Substance” was the name they gave to whatever it is that has the qualities. Latin substantia means “standing under.” Descartes defined “substance” as “that which needs nothing but itself and God to exist.” He considered that there were two kinds of substance (three if you count God), namely mind (res cogitans or thinking thing) and matter (res extensa or extended thing). Substance cannot be perceived, but is known by reason (Meditation II).
The concept of substance is intended to account for at least three things:
1. The fact that the properties of a thing stay together. The lamb in the example has all those qualities and many more; some of them stay together as long as the lamb exists, some for definite but shorter periods of time.
2. The fact that our senses agree. I can touch what I see, and if I strike it it may make a sound that comes from the same place.
3. Intersubjectivity, the fact that what I sense is also capable of being sensed by others.
Note that these conditions are the basic ways we settle a doubt about the existence of something. For example, we ask “do you see what I see?”
Beyond these matters. it is not always obvious what should count as substance. Is a stone, for example, one substance or a collection of them?
The main difficulty of the concept, however, is simply the fact that substances are not directly perceived. How do we know there are substances? The three characteristics I mentioned above are not available because they are what the concept is supposed to explain. Suppose I claimed that my watch keeps time because of an invisible time spirit inside of it. It wouldn’t do to say I know that there is such a spirit there because the watch keeps time.
The strange thing is that even after Berkeley and Hume showed that there is no way of establishing that there are substances, the concept led an undercover life. Philosophers still assumed that they were there even though they were unknowable. This is a part of the reason Hume called himself a skeptic. It also survived in our common belief that there are minds and there are material things, and even in the “scientific” view that there is only matter and that “minds do not exist.”
Note: Aristotle’s translators use the word “substance” in a quite different sense. The two should not be confused.