For the same reason mentioned before. If our brains were not intelligently designed, we could not reach justifiably trustworthy conclusions about any external fact.
Are you just not going to even quote the rest of my post or address it? Or would you rather pretend that all I did was ask "why" instead of covering my actual points of contention?
This is not a controversial or idiosyncratic claim, but a pretty mainstream epistemological observation.
You keep using that word, but I don't think it means what you think it means. Not that philosophy ever has a say in science anyways. In fact, by its very nature, philosophy is never conclusive. However, given that being a theist is more common that being an atheist, it would not shock me if the "mainstream" philosophy reflected a belief in a creator god. However, even though I have been through 2 philosophy courses, what you bring up is entirely new to me, even though those classes did touch quite a bit on religious ideas. Basically, philosophy is a poor choice for trying to demonstrate the existence of anything aside from varied thought processes in people. You can't prove or even provide evidence for much of anything through philosophy. So, mainstream or not, your argument is just your opinion, in the end. I don't think anything had to be designed by a perfect designer to have some degree of trust, yet, you do. So? Your personal views aren't evidence that this designer exists.
Would you trust the calculations of a computer that was not intelligently designed?
-_- I trust granite to be hard, even though as far as I'm aware, nothing designed granite. Since all computers are designed, you made a leading question. That is, any computer that isn't intelligently designed is "unintellegently" designed. That is, designed by a moron, I suppose. Since your question isn't even valid for our discussion, given that computers are always designed, I will compare something that can both be designed and due to random chance: paperweights.
Whether or not something is designed has no bearing on whether or not they function as I expect. A designed paperweight carved from rock and a random rock I pick up off the ground could equally provide a paperweight's service (weighing down papers) by virtue of their weight. However, maybe the designed paperweight's shape was more decorative than functional, so it gets easily knocked over and isn't even as good as the rock, so I never use it. Maybe the designed paperweight is the best paperweight ever and I never use the rock over it. The only difference here is that, with the designed paperweight, I have a designer to complain to or praise, depending on how good the paperweight is at its job. The rock is just a random rock produced by nature, so whether or not it is good at its job is chance rather than design. Thing is, though, designed items are generally terrible for any other purpose than what they are made for. I bet your average paperweight isn't better at hammering in a nail than the average rock I'd pick up to use as a paperweight.
But notice a question you chose not to ask: would I trust the calculations of a well made computer over the individual that designed it? Assuming the designer was human, I'd trust the computer over them. Assuming the designer was "absolutely perfect", the question should be irrelevant as a perfect designer would design things perfectly. Both designer and creation would be equally reliable. Yet, you cannot deny that you see imperfections all around you, whether it be the moles in your skin caused by cellular errors or that your own DNA gets shorter every time it replicates while in bacteria that doesn't happen. The world you assume is designed by a perfect designer is not perfect. The inevitable implication would be that the designer made the design imperfect on purpose. Thus, why would I trust this purposely imperfect design? Heck, why would I even trust a "perfect designer" that would INTENTIONALLY enact imperfect designs?
Simply put, I take everything with a grain of salt. A lack of absolute certainty doesn't bother me, I just go with whatever seems to be most likely. If that ends up failing, and it has from time to time, so be it. I accept that nothing in life is 100% guaranteed, but that doesn't make it entirely random. I'm not likely to open my front door and encounter a black hole, so I have no reason to worry about that. The chances of me opening that door to encounter rain are far more likely. So, it could be considered reasonable to keep an umbrella by the door in case of rain, but not enter and exit through windows because there might be a black hole by the door.