When we talk epistemology we often talk about grounds for belief. But if we are to have a full account of inquiry, we need more than just reasonable grounds for belief; we need reasonable grounds for suspicion. Whereas belief involves assent, suspicion (as in "I suspect that...") involves motivated inclination where we still have to leave quite a bit open to revision, real inclination on weak expectation; and we often inquire based on what we suspect to be the case. I would suggest that all reasonable grounds of suspicion reduce to three (or some combination of the three):
Broad Analogy: First, reasonable suspicion may be based on experience with cases that at least seem broadly or loosely similar; one carries over expectations from such apparently similar cases. In other words, we fill in gaps with our expectations from cases that seem broadly analogous.
Probable Economy: Second, reasonable suspicion may be based on an assessment of what seems to offer the highest probability of (to quote Ernst Mach) "a picture of the world as complete as possible — connected, unitary, calm and not materially disturbed by new occurrences: in short a world picture of the greatest possible stability." Things like 'elegance', 'parsimony', and such go here. In this case we make desiderata for our models of the world (of a kind that our based on our cognitive needs) expectations for the way the world actually is. We prefer models that are easy to remember, simple to use, convenient to apply.
Inkling of Promise: Third, reasonable suspicion may be based on an assessment of what seems likely to be most useful for discovery if true. Unlike economic or analogical grounds, such grounds are wholly practical in character. This sort of reasoning has the structure of a wager; and the expectations are based on the practical goal of facilitating inquiry.
All three of these are apparent: for instance, well-established analogy gives you reasonable opinion or belief, not suspicion. What gives you suspicion is a sort of prima facie analogy, analogy at first glance; and so with the other two. You have suspicion when the assessment involved is done under such circumstances that we can recognize that there are many potentially relevant things left out. My reason for thinking that these three cover all is that inquiry involves our minds on the basis of prior experience engaging in a practical activity geared toward discovery, and every part of this is covered by a ground: our minds (economy) on the basis of prior experience (analogy) engaging in a practical activity geared toward discovery (promise).
What do you think?