The Logic of Surprise — Summary

George Lennox Sharman Shackle’s “The Logic of Surprise” is a compact theoretical essay in expectation theory, published as an Economica note, that asks how a person can coherently anticipate surprise. Its scope is logical rather than anecdotal: Shackle wants to specify what kinds of imagined future make surprise possible. He treats expectation through “potential surprise,” where to expect an event can mean assigning it zero potential surprise; this creates the opening paradox.

Surprise is what we feel when an expectation has gone wrong, has proved in the event to have been ill-founded and false.

The first move is a distinction hidden by ordinary speech. A counter-expected event has been imagined, considered, and rejected; it belongs to the set of possibilities but carries high potential surprise. It may shock us if it happens, yet it is not outside the field of anticipation.

A counter-expected hypothesis, that is to say, is in rough language a hypothesis that has been looked at and rejected.

The second category is stricter. An unexpected event, in Shackle’s technical use, is not merely unlikely; it was never formulated. This rules out the wrong solution to the paradox: one cannot expect surprise from a counter-expected event, because expecting that event would give it zero potential surprise and so cancel its counter-expected status.

In contrast with this I define an unexpected event as one which has never been formulated in the individual’s imagination, which has never entered his mind or been in any way envisaged.

Shackle then asks when genuinely unformulated outcomes can occur. In a yes-or-no or one-dimensional question, the possible answers can usually be surveyed implicitly. The field opens only when the future answer is a schedule of many details and combinations, as with a Budget or a political-economic situation. Surprise depends on omitted dimensions in the agent’s prior image.

Complexity of the event is therefore, I think, a circumstance which must be present if the individual is to be in fact exposed to the occurrence of an unexpected event.

The conceptual center of the essay is the “residual hypothesis.” An exhaustive set of rival hypotheses is one in which the agent believes the true answer will fit under some heading. If his precisely stated hypotheses do not exhaust the field, he must add a heading for what he cannot specify. The residual is thus not a veiled prediction but a disciplined acknowledgement that his articulated possibilities are incomplete.

If so, in order to make the set of hypotheses exhaustive he will have to include a residual hypothesis, which will be simply a recognition of the non-exhaustiveness of his list of precisely stated hypotheses.

This residual heading makes “expecting to be surprised” coherent. The agent may attach zero potential surprise to the fact that the truth will fall under the residual category, while also recognizing that its detailed content will surprise him. Shackle’s imagined nineteenth-century physicist shown an electronic digital computer clarifies the case: all available explanations may be unsatisfactory, so the true explanation is expected only as something beyond the current imaginative repertoire.

For he will then attach zero potential surprise to the hypothesis that the filling of the blanks will, by its manner or matter, not by its occurrence, surprise him; and to attach zero potential surprise to a hypothetical future event is one thing that we can mean by ‘to expect it’.

The closing section applies this logic to economic conduct. In Shackle’s decision theory, complex outcomes can be summarized by focus-gains and focus-losses on a gambler indifference-map. A residual hypothesis cannot be reduced to a single stable value; it tends to require widely separated favorable and unfavorable possibilities. Hence actions exposed to residual ignorance may lie below the indifference curve represented by doing nothing or holding cash. The practical effect is a theory of inhibited enterprise: agents may avoid action not because the news is plainly bad, but because it is too internally conflicting or obscure to make into an intelligible set of hypotheses.

The essay’s thesis is therefore that expected surprise is not contradictory when what is expected is a residual category, not a specified event. Its relevance is to replace a picture of decision among fully listed alternatives with one centered on imagination, blankness, and the awareness of unformulated possibilities. Surprise becomes a logical marker of the limits of the conceived future, and economic inactivity becomes an intelligible response to that limit.