Dashboard Knowledge
By dashboard-knowledge, Barfield means merely operant knowledge of the world: the ability to manipulate and control--to drive--nature, an ability which does not at all depend upon actual understanding of its meaning or purpose.

When we read a speedometer, Barfield reminds, it is well to remember that we are not actually experiencing the speed of the car. Similarly, when we inspect the findings of a radio-telescope, we must remind ourselves that we have only dashboard-knowledge of the nature of the stars.1

When we look at modern science's conception of theory and hypothesis, and its acceptance of the "if it works it must be true" approach, it "is almost as if they expected dashboard-knowledge to tell us how the engine was made" (SA 60). Galileo may have counseled--as one of the foundational principles of the scientific revolution--that "In every hypothesis of reason, error may lurk unnoticed, but a discovery of sense cannot be at odds with the truth" (quoted in WA 26), but in fact much of modern science now has little to do with "discoveries of sense," relying wholly on dashboard knowledge instead.

It was in Poetic Diction that Barfield first introduces the concept of dashboard knowledge in a striking parable:

Once upon a time there was a very large motor-car called the Universe. Although there was nobody who wasn't on board, nobody knew how it worked or how to work it, and in the course of time two very different problems occupied the attention of two different groups of passengers. The first group became interested in invisibles like internal combustion but the second group said the thing to do was to push and pull levers and find out by trial and error what happened. The words "internal combustion," they said were obviously meaningless, because nobody ever pushed or pulled either of these things. For a time both groups agreed that knowledge of how it worked and knowledge of how to work it were closely connected with one another, but in the end the second group began to maintain that the first kind of knowledge was an illusion based on a misunderstanding of language. Pushing, pulling, and seeing what happens, they said, are not a means to knowledge; they are knowledge. It was an odd sort of car, because, after the second group had with conspicuous success tried pushing and pulling all the big levers, they began on some of the smaller ones, and the car was so constructed that nearly all of these, whatever other effect they had, acted as accelerators. Meanwhile the first group held its breath and began to think that their kind of knowledge might perhaps come in useful after the smash. (PD 23-24)
He returned to the analogy in later books. In Saving the Appearances, for example, we find the following:
    Take a clever boy, who knows nothing about the principle of internal combustion or the inside of an engine, and leave him inside a motor-car, first telling him to move the various knobs, switches and levers about and see what happens. If no disaster supervenes, he will end by finding himself able to drive the car. It will then be true to say that he knows how to drive the car; but untrue to say that he knows the car. As to that, the most we could say would be that he has an "operative" knowledge of it--because for operation all that is required is a good empirical acquaintance with the dashboard and the pedals. Whatever we say, it is obvious that what he has is very different from the knowledge of someone else, who has studied mechanics, internal combustion and the construction of motor cars, though he had perhaps never driven a car in his life, and is perhaps too nervous to try. Now whether or no there is another kind of knowledge of nature which corresponds to "engine-knowledge" in the analogy, it seems that, if the first view of the nature of scientific theory is accepted, the kind of knowledge aimed at by science must be, in effect, what I will call "dashboard-knowledge." (SA 55)
Barfield's understanding of dashboard knowledge informs as well the following exchange between Burgeon and the astronomer Ranger in Worlds Apart:
Burgeon: . . . To my mind the very word "radio-telescope"-- and "radio-microscope," too, for that matter--is a hidden lie. A telescope is an instrument for seeing things at a distance. If I understand rightly, you don't see anything at all through a radio--telescope except a lot of squiggles on a screen.
    Ranger: You're a little out of date. Actually I believe the data are recorded on a punched tape, which is then fed into an electronic computer.

    Burgeon: Exactly! You might as well say a man who is looking at a speedometer--no, at a column of figures made up from speedometer-readings, is seeing the speed of the car!

    Ranger: Well, isn't he?

    Burgeon: He's not looking at it anyway. He's looking at a pointer that measures it. (WA 18)

See in particular Saving the Appearances, Chap. VIII. and Poetic Diction, "Preface to the Second Edition."
1"For, while the dashboard of even the most expensive and up-to-date car is a comparatively simple affair, nature's 'dashboard'-that is, her exterior, accessible to the senses and the reason-is of such a marvelous and intricate complexity that many a man has counted his life well spent in mastering a tiny corner of it" (SA 56).