Like you, let me revert to speaking generally so that you can better understand my position. Consider this 'Harv's thesis':
As I sit here typing on this keyboard I can reflect on all that I know about the world. If I think about most of that knowledge, I realize that the majority of my knowledge is in the form of memories. I don't currently see all the books I've read, I don't see all the places I've visited, or much of anything except the room around me.
If I consider those memories and the room around me, I have this strong hunch (almost an unescapable one) that all my memories and the room around me is real. That is, my perceptions and memories accurately perceive this 'real world' to at least a sufficient degree. My memories tell me that other people have had similar perceptions as me, and I conclude that we all pretty much experience this real world that is 'out there'. Could I be mistaken? That is, is it possible that I was born 2 seconds ago with memories intact and that really I am a brain in a vat being supplied with perceptual data? Yes, I can be mistaken. I see that brain in a vat possibility as unlikely, but is unlikely from the data that I have to make such a judgement. If the world was entirely unimaginably different and I was a brain in a vat that was 'turned on' 2 seconds ago, then that 'unlikeliness' could have been programmed as part of my wiring, so even having this feeling of unlikeliness means absolutely nothing.
However, brains-in-a-vat to the contrary, I feel that I have to make certain assumptions about the world in order to make a productive use of my day. If I really believed that I was a brain in a vat turned on 2 seconds ago, I think it would be meaningless to type this post, and I think it would be more meaningless for me to stay inside a room not ever having experienced anything but memories. For obvious reasons I choose to reject such a scenario along with thousands of other scenarios that seem unlikely to me. It is far more satisfying to believe that 'what appears to be' is in fact 'what actually is - at least with a high degree of accuracy'. Therefore, I have my first premise:
(P1) Our perceptions and memories are about a real world that somehow are translated by our brain into a meaningful experience of this real world. If our perceptions and memories are such that the world appears consistent with those perceptions and memories, then our perceptions and memories are 'true' of this real world.
So, here I am, believing that the world around me is real. Believing that my experiences are real. That my memories are real. Next on my list to consider is what we humans actually know and what we simply infer based on invalid grounds. I accept knowledge of my perceptions and memories as real (even though I really don't know that I've chosen to accept it). That being the case, I quickly realize I have another bump in the road. That 'bump' this time is that it is one thing to assume your perceptions and experience is about a real world, it is quite another to assume that our explanations of how the world works is really how the world works. For example, just because I think that the lights above me work on the concept of photons travelling through space to my retina, doesn't actually mean that this is what is happening. I only experience light, I don't know that there are things such as photons, and that photons travel through space. If I think about it long enough, I realize that I don't have any direct perceptions of photons and I have not even one memory of a direct perception of a photon.
This presents a serious problem for any theory of the world. We have perhaps the best of all reasons for believing a theoretical structure to exist, but because I don't have direct perceptions and memories of it, I have to infer the existence of those things. Inference is tricky business. Even simple matters as inferring that there is a quarter in a certain spot in a shallow pool, can fool my perceptions as I go to pick up that quarter. Theory of light suggests that light bends when it leaves/enters water, so using this theory I can better understand how my perceptions can be fooled (without having to give up my first premise to accept my perceptions as experiences of veridical things). Unfortunately, the history of inference is full of pitfalls. Inferences have been wrong, and even more subtle is it that theories have been restated in terms of more general theories such that the more primary theory needed to be revised after it was found that the more primary theory made mistaken predictions. Unlike my first premise, I cannot treat almost all of my inferences as real. What 'appears to be true by inferring' may not be 'what actually is - even with a low degree of accuracy'.
This presents a problem. I have many experiences of good inferences (and theories) where the predictions were uncannily reliable. I also have noticed that using those theories humanity's control of the world has expanded drastically. Whatever position I hold on the nature of theory and inference, I certainly need to account for these successes. I could just reject theory, but that wouldn't jive with my perceptions and memories (1st premise) of all these successes that followed from theoretical inferences. Thus, I see myself being compelled to accept some pragmatist theory of truth such that 'success' becomes primary in considering the question of 'what theories are actually true'.
But, hold on. There's many false inferences and beliefs that yield success. In fact, some of the great discoveries of science were based on false inferences that just happened to lead to great discoveries. So, it can't be any pragmatic account of truth, otherwise we should believe in lucky charms and Michael Jordan's old college shorts (which he wore under his team's shorts in many of his great playoff games), etc. There must be a more specific account of pragmatic success that yields the most appropriate account of the successes of science and which hopefully will give us our most hopeful means by which to look at scientific facts that are based on theoretical inferences.
Well, there is another approach instead of beliefs that match well with successes. We can hold a method of discovery as being more pragmatically useful. That is, we can say that the scientific method generally leads to successful theories. The methods that comprise the so-called 'scientific method' thus becomes one of our top contenders for something that generates truths of the world that we should generally accept as 'true'.
So let's recap. So far we've covered the absolute human need to consider perceptions as veridical. We have also covered how successful theories aid in our prediction and control of the world (as well as our understanding - but that's a separate issue). The best hope of explaining success in science is by accepting a best method that seems highly prone to generating successful theories of the world. The next area is answering why a method can be fine-tuned for success. In short, does a method generate not only success, but does it also generate truth?
What we need is a pragmatic theory of truth. This is a theory of truth that says that we have good reason to believe a theory is true, if it can be shown that the theory is successful. Or, in our special case for a pragmatic theory, a theory is shown to be true if it originates from the so-called 'scientific method'.
Unfortunately, we cannot know for absolute dog-gone sure that perceptions are veridical. Any notion of truth is of course reliant on the 1st premise that I accepted as absolutely necessary. Hence, the truth of the scientific method cannot be any more sure than we are of the first premise. However, it cannot be as sure as the first premise since my perception of a keyboard is a direct experience, whereas theories are inferences which have been wrong. Even the scientific method has generated theories that were successful, but eventually proved to be partly inaccurate. I'm assuming that there being a keyboard in front of me is as highly accurate of a statement (based on the 1st premise).
So, a pragmatic scientific method theory of truth relies, by definition, on success. Success is reliable for accurate prediction and experiences (dealing with the first premise). But, does success have any relationship with truth (i.e., assuming the first premise as a given)? Well, if we accept the first premise, then there is very good reason in accepting a second premise:
(P2) Consistent success of obtained knowledge from a particular method is a postive indication that the conditions of (P1) have been met. Hence, (P1) implies that a successful method of knowledge acquisition provides truthful statements about the real world.
Let me support (P2). Let's assume that a successful method did not give the same sufficient reason to accept the 'truth of a matter' as (P1). This would mean that either a) there is more success experienced in (P1), or that b) there is something about (P1) that we accept as necessary that is not related to success, or c) there is something afflicting (P2) that does not afflict (P1). The problem with (a) is that there is no reason to assume that 'perceptions as true reflections' is any more successful than 'methods as providing true reflections'. In fact, perception is itself a method. If we are temporarily fooled by a perception (such as if we heard our name being called out of error), we immediately try to determine if that perception is real. (P1) is not based on such kind of perceptions. It is based on a full review of our perceptions. We actually try to see how our perceptions are fooled, and we use all of our perceptions to rule out poor perceptions. Who, for example, hasn't been fooled by one perception, but used other preceptions to improve our understanding? Eventually, we are able to resolve any perceptual problems by methodically reviewing our perceptions, obtaining more perceptions, and experiencing some kind of success as a result. (P2) relies on exactly the same process. There is so much success from a successful (P2) method that no rational mind would even question (P2) in many instances. As for (b), what is it about (P1) that doesn't include success? I can't think of any feature of (P1) that isn't influenced by the concept of pragmatic success.
The issue with c) is the possibility that there are errors in (P2) that are not in (P1). I didn't mention too many concerns for perceptual errors in (P1) if we properly check them out, but obviously perceptual errors in (P1) happen all the time. People suffering from brain disease, mental illness, etc are afflicted with errors in their perceptual abilities. Therefore, (P1) is a general statement of truth (not so much a specific statement). Since (P2) is based on (P1), therefore (P2) is also error prone. In (P2) there is perhaps more errors that can occur that don't necessarily include maladies; the issue of being 'close minded' and 'dogmatic' (etc) can also create error. At the same time, a hardnose frame of mind can afflict (P1). People who are prone to hold a certain belief are also prone to misperceptions. Do the effects of various hardnose beliefs affect (P1) as much as they do (P2)? I think the answer is no. (P1) is not as much affected by hardnose-ness as (P2). However, are the successes of (P2) experienced if hardnose-ness gets in the way? For example, if dogmatic beliefs distort one's perceptions of (P1), then likely the perceptual successes achieved by (P1) will also be affected.
Another issue related to distortions caused by hardnose-ness, is that sometimes hardnose-ness results in great success and other times in failure. For example, Einstein was certainly 'hardnose' when he discarded aether, but the result of his hardnose-ness led to something that was fully successful. On the other hand, his 'hardnose' attitude to quantum theory could be considered a failure on his part. In both instances, the hardnose-ness had an effect on the success and no-success followed (in the case of quantum theory he had no success to show for his approach). Sometimes the hardnose-ness is not rewarded until after the theorist is long gone (e.g., I believe Boltzman might fall into that category).
Thus, (P2) follows from (P1), but the complexities prevent any attempt to make (P2) a natural deduction of (P2). Therefore, (P2) remains a premise. A premise that there is very good reason to accept as an overall case (i.e., assuming (P1) is true).
If (P2) is accepted, then we have reason to accept scientific theories as 'true' (at least in a manner similar to how we hold our perceptual experiences as true). There's not an implicit requirement to accept (P2) even if we accept (P1), but I think by the standards of a rational mind, (P2) becomes much more difficult to avoid. Therefore, I conclude that a pragmatic approach to truth with an emphasis on accepting scientific truths is the best policy. I believe that any other policy leads to inconsistencies.
Regarding your contention that we can accept some of (P2) and not all of (P2), I think here you run into trouble. Once we have accepted (P2) where do you draw the line of what (P2) can generate as true and what it cannot generate as true?
Sorry for being long winded, I thought a full account of my perspective would help in showing why I reject your perspective. I don't think (P2) can be so easily divided as you suggest.
Warm regards, Harv