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Why is it the fewest assumptions most reasonable?
A good argument is built on good data and premises. The further we move away from "Justified True Belief", aka knowledge, the more error-prone our conclusions are likely to be.
Accumulating Justified True Belief is the act of reducing potential errors in reasoning.
Justified True Belief is the standard because it is the most consistent way to build an accurate understanding or model of the world.
A Justified Belief can fail to be true for at least 2 reasons: 1) because we lack the evidence or awareness of the evidence to justify the true belief, or 2) because we have chosen poor justifications, such as logical fallacies.
Logical fallacies are reasoning that have provably failed to determine the truth of a hypothesis or belief, and are therefore not useful for separating the wheat from the chaff. Example: "Well Socrates thought the sun revolved around the earth, and he was a smart guy." Fallacy: appeal to authority.
True Belief, while true, is indistinguishable from false belief if it is not justified. Just as many once thought the sun rotated around the earth and believed fire is hot, we can have true and false beliefs. We need justification to separate the wheat from chaff.
While Justified True Belief is currently the best understanding of how to arrive at correct conclusions, even a justified true belief can be wrong. Ever JTB we have should come with the humble understanding that it is only our "current best interpretation".
Why should we want to understand the world better? How is it actually more useful?
Those who have an accurate understanding of the world are more capable at navigating dangers and effecting change. Meteorologists who don't understand what temperatures cause rain to become snow are going to be less effective at their job than those who do.
Aside from being effective, we can simply want to develop a more accurate view of the world for the love of understanding.
Because it helps us be more compassionate. Having an idea of what people are going through allows us to empathize, and can allow us to help them more.