What’s the meaning of “purchase” in “the purchase for changing a rule”?

Question

On page 118 of Thinking Like a Lawyer by Frederick Schauer, I found this passage:

If the purchase for changing a rule[,] is a perception of what the optimal result would be in the absence of the rule, and if every case in which the rule generates a suboptimal result is understood to be one in which the rule should be changed so that it no longer produces these suboptimal results, then it appears as if the rules are not operating as rules at all and that all of the work is being done by the rule-free determination of the optimal result.

I see that purchase is being used as a noun here, yet what does it mean?

None of these three Oxford Dictionaries definitions under noun seem to apply to this context.

I’d guess its meaning here as “reason, rationale”, but I don’t know.

Answer 1

It's meaning 2, 'firm contact or grip', but in a metaphorical sense (i.e. not literal, since there is no physical contact) coloured by the original meaning of the verb 'seek to obtain'. Literal usage, as shown by the examples, typically conveys a sense that the contact is firm only through significant effort (that is, it is something that was 'sought to be obtained').

The author is implying that the optimal result might seem a good reason to change the rule, but that the person seeking to change the rule set out with that as their object and only subsequently identified the optimal result as a post hoc justification - indeed, a 'rationale'.