Epistemic Status: I didn't think about this for that long, could be improved, but I still think it's a good first pass.

This post is was written as an answer to the "and how do we know that?" part of the question Space colonization: what can we definitely do and how do we know that?

Working on questions related to space colonization, I've formed a loose taxonomy of the kinds of arguments I've encountered - primarily those in Eternity in Six Hours, but also other sources and arguments I make myself.

A Taxonomy of Argument Types

1. Our understanding of the laws of physics says it should be possible. (Argument from Physics/Basic Science)

At the most basic, we have reason to believe doing something is possible when applying highly-confident models of physics say that it should be. For example, we have a lot of confidence in the laws of motion, general relativity, and chemistry. Related to these, we have (I believe) a lot of confidence in our models of astronomy and how far away different stars. These models state that with enough energy one can accelerate objects to fast enough speeds to reach remote celestial destinations.

At this level, we're dealing a lot with energy, distances, accelerations and things that resemble back of the envelope calculations.

Usually application of physical theory also requires use of empirically obtained data, such as calculations of interstellar dust densities. However, this data can often be generated from robust models and sensors that work off basic physical quantities. Plus one can explore the sensitivity of the models showing that even changing empirical parameters by an order of magnitude doesn't undermine the overall argument.

Note that the arguments in these models are both contingent and used to rule out of a lot of goals and scenarios. We could imagine worlds where everything is much further away and achievable speeds are much slower such that we could never reach anything. We also don't spend thought on things which seem ruled out, e.g. faster than light travel, or even levels of speed that would require exceedingly enormous quantities of energy.

2. Things nature has done, so reasonably we as intelligent beings in nature should eventually be able to too. (Argument from Nature)

Without elaboration, the authors of Eternity in Six Hours rely on this argument when making the assumption th...