In fact, this is what most philosophers of science have said about the basic concepts of physics. They are all tautologies.

Just as Casper wrote, try defining mass, energy, or force.

Now, you could say the same thing about defining consciousness, with one fundamental difference.

Nobody has ever or will ever experience standalone matter, something purely physical. There is no evidence for such a thing nor could there ever be because the existence of such a thing is an absolute impossibility.

Now, if anyone ever (like Daniel Dennett or Patricia Churchland) tries to tell you, "I am unconscious," then you can conclude they don't understand even basic English.