This one appears fairly exhausting at first look. I’m going to take it at face worth that the Montagues themselves are telling the reality, and due to this fact that there’s just one thief / potential liar, and in addition that the portray just isn’t on the backyard or the pond.

You would possibly begin by searching for two or extra statements that create some type of contradiction, which might point out that one in all them have to be a lie. For instance,

Taylor says that if the portray is on the windmill, then Jessica is responsible. But if the portray is on the windmill, then that makes Beth’s assertion a lie, which might suggest there are two responsible events, which contradicts our assumption that there’s just one. So we would deduce that the portray is NOT on the windmill (or that Taylor is the liar). And proceed from there.

Another strategy is likely to be to imagine the portray is at a sure location, after which work out which assertion(s) could be lies in the event you had been proper. For instance,

You would possibly begin by assuming that the portray is on the boathouse. But then that might imply each David and Karen are mendacity, which once more contradicts our assumption that there is just one liar. So we would conclude that the portray is NOT on the boathouse. Etc.

I believe I’ve discovered an answer which is totally per all the situations of the puzzle, though I need to admit it is a bit unsatisfying and it definitely doesn’t preclude the existence of different options:

It appears attainable that the portray is on the cottage, and the thief is Grant. All of the visitors, in addition to the remainder of the employees, could be telling the reality. Beth the banker begins on the windmill; Charles the gerontologist begins on the cottage; David the entrepreneur begins on the pond; Frank the filmmaker begins on the mansion; Jessica the composer begins on the boathouse; Karen the decorator begins on the lighthouse; and Taylor the lawyer begins on the backyard. I suppose this would possibly suggest that Charles discovered the stolen portray on the cottage however did not say something? There’s loads about this puzzle that is ambiguous so I’m unsure if that is an unreasonable assumption.

UPDATE

Per the remark from @WoomyRogue, I’m assuming that neither the Montagues nor the employees can lie. This makes the puzzle considerably simpler, and I consider I’ve discovered the answer. First,

The portray can’t be on the backyard or the pond (Alistair). If the portray is on the windmill, then Beth is mendacity. This additionally means both Taylor is mendacity, or Jessica is responsible. Either method now we have two responsible events, which contradicts Gordon. So the portray just isn’t on the windmill.

If the portray is on the cottage, then Charles is harmless (Sandy). But then Charles is telling the reality, so he is a gerontologist who began on the cottage. This would suggest Grant is mendacity, which is not attainable. So the portray just isn’t on the cottage.

The thief:

Therefore the portray have to be at both the boathouse, the lighthouse, or the mansion. This means Karen is mendacity, and is the thief.

The location:

By David’s assertion, we all know that the portray just isn’t within the boathouse. By Evelyn’s assertion, we all know that if the portray was within the mansion then the entrepreneur could be the responsible get together. But we additionally know that Karen is the decorator (Lyle), so the portray just isn’t within the mansion. Therefore the portray is within the lighthouse.