The starkest difference between Ronald Reagan and Walter Mondale is not so much political as epistemological, having to do with the way one knows things. Walter Mondale -- good old square, predictable Norwegian Walter Mondale -- is entirely familiar to us. We know exactly how his mind works. He holds out no surprises at all.

But even after four years, most of us still do not know Ronald Reagan well. We need interpreters and guides to the terrain of his mind, and even then we are left short of full understanding. It is not simply that people may disagree with him on this or that. Many who support him find themselves regularly surprised and puzzled by some of the things he says.

It is a paradox that someone who is known widely as the Great Communicator has communicated relatively so little about the most important aspect of himself, the way he thinks.

What he communicates, surpassingly well, are particular messages or packages of feelings, mostly assurances; this appears to be a major secret of his political success. What he does not communicate is the logic of his thought. Instead we often receive packages of statistics, observations and proposals, oddly linked.

Sometimes, as the president confessed Sunday of his idea of offering American "Star Wars" technology to the Russians, these packages have not been "roundtabled." Others might say he is acknowledging that the idea is half-baked, that it has not been thought out or rigorously examined. The president makes the acknowledgment, however, without embarrassment. It is his way of not being held accountable for not knowing all the implications and details.

Look at that Star Wars idea. On the face of it, someone who came along and offered our most exotic military technology to the Russians would risk being locked up for espionage. Mondale was amazed, and for good reason, at the thought of transferring such precious knowledge to a foe.

But that is not the worst of it. Nothing we know about Reagan is consistent with his espousal of parity in defensive arms, whether parity is achieved by our gift or their gain. This is a man who does not trust the Russians and who feels we need an edge or a hedge to be secure. How could he, of all political leaders, give up that edge and that hedge? How does this inconsistency escape him?

His Star Wars idea is more than idealistic, which is a good and necessary thing in a political leader. It is sentimental, which is a sign of weakness, of an insecure grip. It reveals the side of Reagan that is neither hawk nor dove but a pussycat, someone roaming idly through a dream world. It may impart a glow, but it cannot be a solid guide to policy.

To reach for this kind of flimsy theory as the justification for a controversial project demonstrates more than the shortcomings of Star Wars. It demonstrates the softness at the core of Ronald Reagan.

His concept of a nuclear Armageddon is more of the same. There is an initial tendency not to take Reagan too literally because the concept seems so far out and because it seems, well, impolite to one's elders to call attention to something that can only embarrass them, like a gravy spot on the vest. Not so long ago I cited writer Ronnie Dugger's exploration of this matter as evidence of a political bias against the president. Now I repent.

For here was President Reagan on Sunday lending genial respectability to the notion of the coming of Armageddon, saying with equanimity that it might arrive in 1,000 years or "the day after tomorrow," and throwing in only the relatively mild reservation that he had never "seriously" planned on it.

One wants to be tolerant, and not to get too grim. But what is the right attitude to take to a president who has blurred the distinction between Biblical prophecy and contemporary reality and holds so loosely a concept so frightening?

He has put himself in a place where it is impossible to chase him out by means of conventional logic and argument, and where anxious emotional attacks appear excessive and out of key. It seems unfair, and yet this is the man unlike all of the rest of us: he has a finger on the button.

For a while I thought about Reagan that he was perhaps an original, someone who, though relatively untutored and undisciplined in a conventional way, might claim to have a certain intuitive feel for things and who in any event had a capacity to put the pieces together in his mind. Certainly this is the earnest and constantly reiterated testimony of some of his closest associates.

But the simpler and truer explanation may be that he doesn't understand some things very well.