I am by inclination a rationalist. That is to say that I tend to side with rationalists in their recent (last 500 years) argument with empiricists. Empiricists suppose that all significant knowledge comes from observation. Everything we know we learn from experience. Rationalists acknowledge that reason without observation is largely useless; however, we think that we know a lot of important things, and maybe all the most important things, by reason alone.
For a simple example, get a piece of paper and draw a simple right angle: two rays extending from a common end-point. Draw an arc near the end-point, centered on the end point. Now draw a second arc, also centered on the end-point but further out from the end point. Guess what? The second arc is longer than the first. You might suspect that you have learned this by observation, but that is clearly wrong. What you have drawn on paper are mere symbols of ideas. You probably haven't drawn it at all but merely thought it. You don't have to go out and test what you have learned; you know it a priori. Always and everywhere, anything you physically construct will obey these rules.
That this knowledge about angles and arcs is not merely abstract, but governs facts in the empirical world is evident from something we have just explained: how a flyswatter works. Why, you might wonder, is it easier to kill a fly with flyswatter than with your hand? After all, the head of the flyswatter is extended out from your arm, at a greater distance from the object of your affection. Since your hand is closer to the fly, why isn't it easier just to palm the darn thing?
The geometry above provides the answer. When you deploy a flyswatter, your wrist moves over a small arc and the killing surface of the flyswatter head moves through a much larger arc. Since both movements take the same amount of time, the flyswatter head has to be moving much faster than your wrist. That is what rationalists call a necessary truth. Reason now trumps biology. I have no idea how a fly brain works, but I can conclude, with the help of experience, that the "fly!" schema in its brain is faster than my wrist speed but slower than the swatter head speed. The real, observable world is governed by a lot of rationally knowable, a priori truths.
Rationally knowable truths are not limited to geometry; they also are available in economics. If you borrow, say, more than a trillion dollars a year to cover your consumption of wealth, then either you have lenders somewhere able and willing to lend you enormous sums indefinitely, or you are going to have to someday pay that money back, or you will have to default on your loans. Empirical knowledge suggests that the first is unlikely.
If you are going to have to pay back the loans, then either your resources will have to grow sufficiently to cover both the payments and future consumption levels to which you have become accustomed, or you will be poorer in the future because of the borrowing you are doing now. This looks rationally knowable to me. Reason alone can't tell me whether the US economy might grow in the near future at a rate sufficiently robust to cover present borrowing, but you might think that the folks signing off on the borrowing would want to present evidence to that effect. Have they? No. They aren't even thinking about it. That is sheer irresponsibility.
A lot of the left (and a recent commenter on this blog) have argued that austerity is bad during an economic recession. "Austerity" here means trying to bring spending and revenues into balance. They have a case. Austerity surely depresses economic growth and that means less revenue coming in and that makes balancing the books all the more difficult. Perhaps austerity is a self-defeating policy. Yet, if that is true, then what is the alternative? The alternative is to keep borrowing to prop up consumption. That means that there has to be someone to borrow the wealth from, no? Is there sufficient wealth somewhere in the international system, some nation or group of nations that are running a surplus sufficient to cover their own consumption and pay for all the nations that are running deficits?
If something can't go on forever, then it won't. If something can't go on much longer, then it will end before much longer. Those strike me as necessary truths. To ignore them would be irrational.