As Hilary Putnam once said, "It ain't obvious what's obvious." Or as I like to say, "One man's datum is another man's theory."
But is it obvious that it ain't obvious what's obvious?
It looks as if we have a little self-referential puzzle going here. Does the Hilarian dictum apply to itself? An absence of the particular quantifier may be read as a tacit endorsement of the universal quantifier. Now if it is never obvious what is obvious, then we have self-reference and the Hilarian dictum by its own say-so is not obvious.
Is there a logical problem here? I don't think so. With no breach of logical consistency one can maintain that it is never obvious what is obvious, as long as one does not exempt one's very thesis. In this case the self-referentiality issues not in self-refutation but in self-vitiation. The Hilarian dictum is a self-weakening thesis. Over the years I have given many examples of this. (But I am now too lazy to dig them out of my vast archives.)
There is no logical problem, but there is a factual problem. Surely some propositions are obviously true. Having toked on a good cigar in its end game, when a cigar is at its most nasty and rasty, I am am feeling mighty fine long about now. My feeling of elation, just as such, taken in its phenomenological quiddity, under epoche of all transcendent positings -- this quale is obvious if anything is.
So let us modify the Hilarian dictum to bring it in line with the truth.
In philosophy, appeals to what is obvious, or self-evident, or plain to gesundes Menschenverstand, et cetera und so weiter are usually unavailing for purposes of convincing one's interlocutor.
And yet we must take some things as given and non-negotiable. Welcome to the human epistemic predicament.
Some opponents of the death penalty oppose it on the ground that one can never be certain whether the accused is guilty as charged. Some of these people are pro-choice. To them I say: Are you certain that the killing of the unborn is morally permissible? How can you be sure? How can you be sure that the right to life kicks in only at birth and not one minute before? What makes you think that a mere 'change of address,' a mere spatial translation from womb to crib, confers normative personhood and with it the right to life? Or is it being one minute older that confers normative personhood? What is the difference that makes a moral difference -- thereby justifying a difference in treatment -- between unborn human individuals and infant human individuals?
Suppose you accept the general moral prohibition against homicide. And suppose that you grant that there are legitimate exceptions to the general prohibition including one or more of the following: self-defense, just war, suicide, capital punishment. Are you certain that abortion is a legitimate exception? And if you allow abortion as a legitimate exception, why not also capital punishment?
After all, most of those found guilty of capital crimes actually are guilty and deserving of execution; but none of the unborn are guilty of anything.
My point,then, is that if you demand certainty of guilt before you will allow capital punishment, then you should demand certainty of the moral permissibility of abortion before you allow it. I should add that in many capital cases there is objective certainty of guilt (the miscreant confesses, the evidence is overwhelming, etc.); but no one can legitimately claim to be objectively certain that abortion is morally permissible.
There is much popular confusion concerning the topic of relativism. One fallacy I exposed earlier, namely, the mistake of thinking that Einstein's Theory of Relativity implies either moral relativism or relativism about truth. Even more widespread, perhaps, is the notion that one who opposes relativism about truth must be a dogmatist. But there are two distinctions here and they must not be confused. One is the distinction between relativism and nonrelativism, and the other is the distinction between fallibilism and dogmatism. The first distinction has to do with the nature of truth, while the second pertains to the knowledge of truth.
In his magisterial Augustine of Hippo, Peter Brown writes of Augustine, "He wanted complete certainty on ultimate questions." (1st ed., p. 88) If you don't thrill to that line, you are no philosopher. Compare Edmund Husserl: "Ohne Gewissheit kann ich eben nicht leben." "I just can't live without certainty." Yet he managed to live for years after penning that line, and presumably without certainty.