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From what I understand rationalism allows some knowledge to be acquired innately, and that rationalism was created as the opposite of empiricism. Since the scientific method assumes that knowledge is acquired empirically, how would a rationalist justify the scientific method?
If a rationalist can't justify the scientific method, then how would they accept scientific laws and theories?
It may be a misnomer to say rationalism was created to oppose empiricism, but I see what you are getting at.
All rationalism claims, is that some knowledge about the world comes a priori - either innate or by thought alone. This doesn't say all knowledge has to come from thought. So, it is kind of agnostic about the scientific method here.
Descartes tried to reason everything from god, he tried to prove god's existence from thought alone, then from there he argued that a loving god wouldn't lie about the nature of reality and therefore we can indeed trust our senses and study the world systematically.
Essentially, you would have to justify, a priori, that the natural world has some sort of regularity after that science works fine.
A more interesting question, is how an empiricist would justify the scientific method - Hume said some interesting things on causation and induction that a lot of modern science seems to sweep under the rug.
First off, rationalism predates empiricism. Rationalism is arguably the normative mode of philosophical enquiry all the way back into prehistory; clearly (for instance) most of ancient Greek philosophy falls closer to the rationalist camp than the empiricist. Empiricism was a late Western technique developed to deal with some of the failings and foibles of poorly implemented rationalism, particularly as it manifested in Christian theology.
It's unfortunate that dogma overtook philosophical rationalism in certain theological circles, unfortunate that such dogma has been wielded a a political tool, and unfortunate that empiricists felt they needed to turn the political tables by rejecting rationalism whole cloth (as opposed to simply dismissing dogma), but such is life.
With that in mind, rationalists generally have no problem whatsoever with empiricism. Empiricism (to a rationalist) is merely another form of rationalist enquiry, one that leverages material observations about the world in order to justify, examine, and expand internal, subjective — i.e. rational — concepts. A rationalist would point out that neither induction nor deduction is empirical in nature, though both are essential aspects of empirical research. There is no 'evidence' that induction or deduction is 'true'. These are cognitive concepts that we have established through pure application of reason. Even falsification is non-empirical. Falsification is nothing more than a rule we imagine and apply to help us deal with anomalous results.
The place where rationalists start to get fidgety is where empiricists essentialize concepts as real-world objects. For example, consider the Theory of Gravity. There is a natural tendency to treat the Theory of Gravity as though it is an objective feature of the external world, independent of any observer. But at the same time, it is fairly obvious that the Theory of Gravity is merely a mathematical model that exists nowhere except inside of human heads. It's a good model, meaning there is a high correspondence between what we imagine will happen and what we actually see happen; no one has much trouble making the induction from observed events to the theoretical model. But empiricists typically make a second induction — from the theoretical model to some real-world object — that rationalists are typically leery of. Put more simply, empiricists and rationalists can both see a red ball, and can both accept that there is something out there that corresponds to the red ball they both see. But empiricists are prone to say that the 'red ball' is out there in the world, while rationalists tend to be more skeptical, holding 'red' and 'ball' as cognitive constructs that we have applied to the thing, not qualities inherent in the thing itself.
Rationality is essentially human logic plus empirical facts. Science is essentially a systematic and rigorous application of human rationality. The scientific method is therefore rational. Thus, rationality precedes science but science is designed to perform better.
There are several important empirical facts concerning rationality. First, it is possible to distinguish between rationality and irrationality. Second, we can often assess whether our actions produced a bad result or a good result. Third, rationality seems to work better than irrationality. Fourth, where it is used, science often can be seen to work better than ordinary rationality.
Thus, rational people accept that science is often the best way to be rational. This is not always true, though. Ordinary rationality is good enough for our everyday life. To use the scientific method in every circumstance of your life would be silly because in many cases science wouldn't give results fast enough or even wouldn't be able to give any result.
And this is all there is to it. Use what works best.
We may wonder that rationality should work at all. Well, when we were ignorant of the science of that, the puzzle was legitimate. Nowadays, we understand why rationality works. Essentially, rationality is a natural capacity we inherited through our DNA, capacity produced by natural selection. Natural selection eliminates characteristics that don't work well enough. We have the logic we have because it is the logic that was good enough to allow our ancestors to survive in their natural environment. If in the past the logic of some people was not good enough, they found it difficult to survive and reproduce. They didn't pass on their logic to us precisely because their logic was not good enough. Thus, it is no surprise that rationality works.
Thus, since rationality works, it is no surprise that science should work, and should work better since it is only rationality on steroids: more rigorous, more collaborative, more organised, more systematic, less forgetful, and often given more resources.
This is all a rational mind needs to feel confident about science.
Rationality does not require that we should be able to explain the whole of reality to accept that science works best, at least in some very specific domains. Rationality only requires that we should use the means best suited to our objectives. Science does not pretend to explain reality. Instead, it is our best method to predict the outcome of real processes, and therefore predict future events. Thus, we should use science whenever we need a method to predict such events.
I think everyone here is thinking too rigidly and too intently on definitions (as most philosophers do so bravo).
Rationalists believe that reason can explain everything. In other words, everything that can and will happen has an explanation that logically produces the result. The problem that I think many are having is they assume that rationalists make the assumption that anything can be predicted within the mind. While I would be remiss to say that isn't true, it is also certain that rational reasons for an event have no purpose if that event never occurs. The human mind can imagine plenty of things, but many of them are not applicable to the real world. It is predictable and useful to believe that there are some fundamental truths to the universe. However, just because they exist, does not mean they aren't arbitrary and independent of each other. For example, the gravitational constant (assuming the current knowledge on gravity is even remotely correct) could have been anything else, and just about the same amount of predictability would have come from knowing it. Because this "fundamental truth" could have been anything, there would be no way to predict what it would be for a mind without sensory input. Perhaps it could have been guessed or all the possibilities considered, but the vast majority of the possibilities would be useless to think about for the universe we perceive, and picking the right one with absolute certainty would be impossible without being able to experience it somehow.
To further explain, it isn't so far fetched that just as there are logical truths, there must be physical ones as well because both the logical and physical worlds are assumed to exist (if they didn't, then there would be no purpose to reason about them so it's better to assume they do). The logical world and physical world consist of different things so it makes sense that there must be rules that differ between them. Even if physical truths defer to logical truths (which may or may not be true), the physical truths have no impact on the logical world since the logical world is not physical. For example, gravity can be used for reasoning, but still does not change the fact that (A -> B)^A leads to B. However, this is a bit of a double-edged sword because while physical truths may be logical, there is no logical input to support them. The logical world doesn't inherently propose a gravitational force exists (or at least I can't reason that it does). There must be some physical input to start if I am to ever even think about reasoning about it. At the very least, if I were to reason about something I had never perceived, I would quickly lose interest because of its perceived inapplicability.
To me, the scientific method is a way to narrow down the possibilities of the physical truths that would not be predictable just from reason. Perhaps this makes me not a true rationalist, but I think it would be difficult to find somebody who inhabits either extreme of the spectrum. This makes your question not relevant to reality, much like asking why unicorns love carrots. They don't exist (or at least I haven't seen any) so it doesn't matter why. In any case, I believe that there is no point to reason if it cannot be applied to something "real". This inherently links my reasoning to what I perceive. Whether or not what I perceive affects the process of reasoning is not important. What is important is that what I perceive affects what I reason about. Maybe it is different for others, but I expect that it is fairly similar unless they are unable to sense anything.
Last but not least, never forget that nobody can be perfectly rational about everything. Even if a true rationalist exists, it is fairly likely that the flaws in their thinking are what compensate for their extreme opinion or these flaws allow them to act differently from a rationalist at times.
It doesn't exclude eachother rationalism allows constructs of the mind to find logic and structure in thought
