Hi Richard. I'm afraid, in my case, the answer is 'no'. That may be a shortfall in my comprehension, of course. But I'm confused by both the structure and the content.
I'm not sure why there's a comma in the middle of what seems to be a straightforwardly structured sentence where the 'dalliances' are the subject.
But leaving the punctuation aside, I confess I don't know exactly what the sentence is saying.
A 'dalliance', as far as I'm aware, is defined as a brief, usually non-serious romantic relationship or, alternatively, a frivolous, playful activity. So I take 'dalliances of the mind' to be light-hearted imaginings, whimsical, and possibly of an erotic nature.
Then we are told that these should be 'left to their own devices'... and this is where the sentence loses a bit of sense for me. To be 'left to ones own devices' usually means to be abandoned by others, often with the implication of having to make one's own entertainment or engage in a solo activity. But to be 'left to ones own devices' implies agency and a person being left. I'm not sure that thoughts can have such agency and the phrase would better apply to the person thinking them or doing the imagining.
To me, if you keep the 'dalliances' or imaginings as the subject of the sentence, the second half has to reflect their nature as the products of someone's mind - possibly our narrator. And the 'leaving' of them would require the narrator (or, vicariously, us) not doing something with them. So something like...
'The dalliances of the mind are best left unexplored...' might convey what you're after perhaps?
Alternatively, if it's the 'left to (their) own devices' that's the bit you want to keep, as suggested above you'd need to have that refer to the person doing the imagining. So something like...
'Such dalliances of the mind are best enjoyed when one is left to ones own devices...' suggesting their essentially private nature.
Some variation of either of the above would make more sense to me, I think. But, of course, might not be your intended meaning at all! In which case do please ignore me! 😁
Yes, I agree with Jon. I'm assuming you mean to not consciously play around with the thoughts or try to analyse or organise them. If so, I prefer Jon's first suggestion or something similar, eg. are best left well alone/to wander at free will, etc.