1.6. The Task of the informant
1.6.1. Problems with dealing with 'simple' acceptability

Viewed this way, asking the informant about the acceptability of a given sentence amounts to asking, though obviously not in these terms, whether or not s/he can come up with a set of items N from the mental lexicon that would yield an LF-PF pair whose PF representation is non-distinct from the presented sentence α. For the ease of exposition, I shall henceforth refer to an LF-PF pair whose PF representation is non-distinct from the presented sentence α simply as the LF-PF.
The purpose of asking the informant to report his/her judgment on the acceptability of α is to find out whether there is an LF-PF pair corresponding to α. In order for the informant judgment to be significant, it must be a reflection of properties of the Computational System. And that means that we must make sure that (i) the "Not acceptable" judgment is due to some grammatical condition not being satisfied in the course of the "attempted" derivation―resulting in the failure to yield the LF-PF―and (ii) the "Acceptable" judgment is due to the existence of an N that would result in the LF-PF. It is possible, however, that the informant judges α to be unacceptable because s/he fails to come up with an N because of parsing difficulty. Likewise, it is also possible that the informant judges α to be acceptable because α is 'intelligible' (i.e., understandable) in some way even if there is no N that would result in the LF-PF. And, there does not seem to be a principled means to exclude such possibilities. If the reported "Unacceptable" judgment is due to parsing difficulty, the unacceptability in question would not be revealing about properties of the Computational System. The same holds if the reported "Acceptable" judgment is due to the 'intelligibility' of α. We are thus led to conclude, on the basis of these conceptual considerations, that it is not clear, in principle, what significance or how much significance can be assigned to the informant judgment on simple (un)acceptability of the presented sentence.

1.6.2. Solutions: Invoking a dependency interpretation

One way to maximize testability (see (1b)) is to consider hypotheses whose consequences are in principle testable in any language. Given the diverse differences among languages with respect to lexical items, it seems reasonable, for the purpose of maximizing testability, to turn our attention to some structural (i.e., hierarchical) property at LF, abstracting away from idiosyncratic lexical properties as much as possible. As noted above, the only structure-building operation assumed in the model of Computational System adopted here is one that combines two objects to form one, called Merge. Suppose we hypothesize that a certain intuition pertaining to two linguistic expressions a and b arises in the mind of the informant only if what corresponds to a and what corresponds to b at LF are in a structural relation directly definable in terms of the application of Merge such as: something stands in a structural relation R with something else if and only if the former Merges with something that contains the latter. To facilitate the exposition, let us refer to the intuition in question as γ(a, b).

We can try to avoid the problems noted in the preceding section by checking the informant judgment on the acceptability of α where the interpretation of α includes γ(a, b). What the informant is now being asked, though surely not in these terms, is whether s/he can come up with a set of lexical items N that would serve as input to the Computational System so as to yield an LF-PF pair such that (i) the structural condition for γ(a, b) would be satisfied in the LF representation, and (ii) the PF representation would be non-distinct from α. For the ease of exposition, such an LF-PF pair shall henceforth be referred to simply as the intended LF-PF.

We can greatly reduce the possibility that the informant's "Unacceptable" judgment is due to the parsing difficulty, by having the informant judge the same surface string as the presented sentence α, but crucially not involving γ(a, b). Let us call such a surface string as α'. The informant's "Unacceptable" judgment on α under γ(a, b) should not be due to parsing difficulty if the same informant has accepted α', identical to α. We can also have the same informant judge a surface string α'' that differs from α as follows: α'' is comparable to α in terms of its structural complexity; but there is hypothesized to be an LF representation corresponding to α'' (but not corresponding to α), in which the structural condition for γ(a, b) is satisfied. To the extent that the informant judges α'' to be acceptable under γ(a, b), we can reasonably conclude that the unacceptability of α under γ(a, b) reported by the same informant is due to the failure of the hypothesized condition for γ(a, b) to be satisfied in the LF representation corresponding to α.

Let us further note that another way to substantially reduce the possibility that the informant's "Acceptable" judgment is based on the 'intelligibility' of the presented sentence α is to have the same informant judge a sentence α''' that minimally differs from α with regard to whether the structural condition for γ(a, b) is satisfied; the condition is hypothesized to be satisfied in an LF representation corresponding to α, but not α'''. Suppose that the informant has judged α under γ(a, b) to be acceptable. Insofar as the same informant judges α''' to be unacceptable under γ(a, b), it seems reasonable to assume, on the basis of the informant's rejection of α''', that the informant understands what is meant by γ(a, b), and that her/his "Acceptable" judgment on α under γ(a, b) is indeed based on the LF representation corresponding to α satisfying the structural condition for γ(a, b), rather than being due to the mere 'intelligibility' of α. The relevant points will be elaborated and will be illustrated with a number of concrete examples in subsequent chapters, where we will discuss not only the structural condition for γ(a, b) but also the lexical condition(s).

In sum, by invoking γ(a, b), we can considerably reduce the problems noted in the preceding section concerning the significance of the experimental result. The general design of our on-line experiments is in accordance with the above considerations. In a later chapter, we will address in some depth how the specific aspects of the experiment are 'constructed' and how the experimental results are interpreted in accordance with the heuristics in (1).