Here is a case in point: I have just been having a conversation with some of my empiricist friends (a conversation much like many others I have had in the past) in which they are trying to convince me that logical reasoning cannot provide any knowledge of the world beyond that which can be produced by empirical means. My basic argument to the contrary--that logic and reasoning can provide information about reality beyond that which empirical methods can deliver--can be found here and here. (My position is called rationalism--not to be confused with the sort of rationalism that means a denial that God is necessary to explain or understand the universe, mine is a more technical, epistemological use of the term, such as that discussed here.)
I provided to my friends an example of how rationalistic methodology can provide information about the universe not available by empiricist methods: Take the concept of an infinite past--the idea that there was no beginning to the space-time universe, that the past extends forever back into, well, the past, that an infinite amount of time has already passed as of this present moment. Empirically, we probably don't have the means necessary to determine whether or not the past could be infinite. To know that, we might have to have seen the beginning of the universe ourselves, or at least to have seen many other universes begin and so make some kind of inductive argument that it is highly likely that this universe has begun as well.
However, an analysis of the basic concepts involved in the idea of an "infinite past" provides us with further information here. When we look at the concepts of "infinite" and "past," we can see that the two are contradictory. The concept of "infinite"--as used in this context--involves the idea of an unlimited number of something. So an infinite row of fence posts, for example, would mean a row of fence posts where there are an unlimited number of fence posts in the row. Now, one of the inherent ideas involved in the concept of "infinite" is "untraversability." It is impossible to traverse an infinite, because, by definition, whatever one can traverse is necessarily finite (limited). If I were to propose that I am now about to begin a journey of walking past all the fence posts in my infinite row of fence posts, when would you expect my journey to be complete? Never, of course. The journey could never be complete, because if it ever was, if there ever came a time when I had completed my task of walking past the infinite number of fence posts, it would prove that the number of fence posts was not infinite after all but finite. One cannot ever finish walking past an infinite number of fence posts. Infinites are inherently untraversable.
Now let's look at the concept of the "past." What is the "past," exactly? The idea is that the space-time universe is on a timeline, and that we are currently at some point on the timeline (that current point would be, of course, the present), and that part of the timeline extends ahead of us and has not yet happened (this would be, of course, the future) while another part of the timeline extends behind us and has already occurred (this would be, of course, the past). We can see, then, that the concept of the "past" inherently involves the idea of "traversability"--and more than that that, it inherently involves the idea of "having in fact been traversed." That is, the past has already been traversed by the universe. The universe has already been through the whole past and all its parts. The past has been traversed.
Now we can see clearly why the past cannot be infinite. Since "infinite" necessarily involves "untraversability" while the "past" inherently involves "traversability" and even "having been traversed," the idea of an "infinite past" would inherently involve the idea of an "untraversable traversed thing"--a clear contradiction. The concepts of "infinite" and "past" necessarily, logically, exclude each other. Therefore, there can be no such thing as an "infinite past." Therefore, the past cannot have been infinite. Therefore, it is finite. Therefore, there is a beginning to time and thus to the space-time universe. (Of course, this observation brings up more interesting questions that I won't go into here but which I go into in chapter three of Why Christianity is True, particularly the section on "Deeper Philosophical Issues.")
Now, I shared this example with my empiricist friends, and here is basically what they said. No, one better: Here is exactly what one of them said:
Just because we have accepted clear definitions of words does not mean we actually have a correct or comprehensive understanding of the concept the word seeks to represent. So while your logical argument about "past" and "infinite" being mutually contradictory seems logical, how do you know those are accurate reflections of reality to begin with?This response provides a clear example of the sort of confusion that tends to plague empiricists. Basically, what this person is saying is this: "OK, so you have a clear idea of what you mean by 'past' and 'infinite' and can see that those concepts are contradictory. But how do you know that your concepts of 'past' and 'infinite' line up with reality? How do you know that your concept of the 'past' and your concept of 'infinite' match the reality of these things?"
Our entire understanding of space-time has been utterly revolutionized at least once and further discoveries will likely alter our knowledge further.
This is just like if I had said that red and blue exclude each other so that to the extent that something is red it cannot also be blue, and the response had been, "Well, sure, your concepts of 'red' and 'blue' exclude each other, but how do we know that real red and blue fit into your concepts of 'red' and 'blue'?" How do we know? We know because if something doesn't fit into my concept of "red," for example, then, by definition, it isn't red! It is absurd to speak of red things that might not fit into the concept of "red." If these things do not fit into the concept of "red," that is the same as to say they are not red! The problem here is that the meaning of the word "red" is being forgotten. The arguer is confusedly thinking that when he mentions real red things, he has somehow moved beyond the confines of the word "red." But of course he hasn't, since he is still using the word. As long as he is going to keep talking about "red things," he is necessarily bound to the concept of "red." His "red things" must be "red"--that is, they must fit into the concept of "red." The arguer is thinking he can keep using the word while transcending and abandoning its essential meaning.
The same thing is going on with regard to the concepts of "past' and "infinite." My friend thinks he can talk about some real past or some real infinite that goes beyond and might be essentially different from the meaning of the concepts of "infinite" and "past." But since he is still using the words, he cannot escape the basic meaning of the concepts. Just as we cannot meaningfully talk about red things that don't fit into the essentials of the known category of "red", so we cannot meaningfully talk about an "infinite past" which does not fit into the essentials of the categories of "infinite" and "past". The problem here is a simple failure to keep a clear definition of terms, one of the most basic elements of clear thinking. You cannot keep using a word while leaving behind its essential meaning. Of course, you can redefine words if you want. We could redefine "infinite" to mean "really neat" or something like that, and then I will grant that there is no logical problem with having an infinite past. But that is, in fact, not what the word "infinite" normally means, and it is not the way I am using it in my original example, and so it is equivocation to change its meaning without notice. It certainly would also be permissible to say something like this: "OK, so we can't have an infinite past. But maybe there is something out there which is not an infinite past, because it doesn't fit the definitions of 'infinite' and past'--let's therefore call it a zondula, since we don't know anything about it--and maybe it can exist." OK, I'll grant that maybe zondulas exist, since I have no idea what a zondula is. But now we've simply changed the subject and ignored my point--which is that an infinite past cannot exist.
Certainly, there is no doubt much still to be learned about the universe, the things in it, how it works, etc. But our words and concepts have definite meaning and content, and as long as we are going to refer to them, we cannot consistently simply abandon or forget that definite meaning and content. We do know what we basically mean by "past" and "infinite"; otherwise, these words would be meaningless gibberish, like "zondula." But they aren't, and so their actual content must be remembered when we use them.
Empiricism survives only so long as basic semantic confusion and the sloppy thinking that accompanies it survives. Once clear thinking and clear defining of terms comes to be practiced consistently, empiricism must disappear and rationalism will take its place.