Either you repeat the same conventional doctrines everybody is saying, or else you say something true, and it will sound like it's from Neptune.

Chomsky is highlighting how strongly “conventional doctrine” shapes what counts as reasonable speech in public life. If you merely echo familiar, socially approved claims, you sound normal; but if you state something accurate that conflicts with entrenched assumptions—especially about power, media, or policy—it can seem bizarre or alien, “like it’s from Neptune.” The line points to the way ideological consensus can make truth appear implausible, not because the truth is obscure, but because audiences have been trained to treat certain realities as unthinkable. It’s also a warning about intellectual conformity: originality is not the goal; clarity and truthfulness are, even when they invite ridicule.