>A machine could emulate consciousness in the sense that we could probably in principle build a machine that acted, viewed from the outside, as if it were conscious. But we have no way to measure whether something is conscious or not so we would never really know if the machine actually was conscious, had interiority, had qualia, etc. (three different ways of saying the same thing).

You could ask try asking it? You can ask clever questions that would prove things like persistence of thought in between questions, and ability to reason about things. If it's able to reason about it's own processes, it's basically conscious by any measure that matters.

>There is no good reason to believe that consciousness is reducible to physical phenomena.

Lol, what? Other than the fact that we have millennia of scientific progress that have, in every single area of exploration developed predictive models of reality that have been tested, confirmed, retested and refined over time, and every single one of those successful models is not based on some invisible unmeasurable magic nonsense. What on earth could possibly make you think that consciousness is somehow the sole outlier that doesn't obey the laws of physics and is therefore reducible to physical phenomena?

By the same argument I'll never know if other humans are conscious.

This is clearly the case today, pending some new discovery or intellectual leap surrounding consciousness. For all I know, you are unconscious, and your purpose was to type that comment so as to trigger my realization I am the only conscious being in existence :)

(Not a good way to live your live, but a theory consistent with the evidence.)

And that’s why solipsism is not entirely defeasible. You can’t be certain other people are conscious. But it’s reasonable to think they are. It gets harder with other animals, and even worse with machines. See Ned Block’s paper on The Harder Problem and Commander Data.