It is a “beg the question” fallacy (aka circular logic) because I have refuted your premise that federal roads have no Constitutional basis. You even admit as much when you can’t point to the authorizing text in Article I, Section 8.
The difference between us and you is that we believe in obiding by the actual text of the Constitution and like Jefferson oppose your view that we should rely on Supreme Court decisions based on “stare decisis” (using bad decisions of the past to justify more bad decisions).
Like Jefferson, I believe that our federal government should abide by the Constitution as written by our founders and not the Constitution as imagined by un-elected and un-accountable judges.
“I have refuted your premise that federal roads have no Constitutional basis.”
I think you’re slightly confused. I used the federal roads argument as an example of something that is constitutional, yet isn’t expressly stated in the Constitution. “Post Roads” would have to be expanded upon quite a bit to apply to the federal highway system and the federal funds used to pay for state roads and highways.
Just an example of how Libertarians “strictly” define only when they choose to on “certain” issues important to them, like drugs.