As per usual. Context. Which we don't have. It accepts 'thin' in any case. I suspect the use of it for perceptive (if you want to be clear use perspicace) is in fine/fin's meaning as keen or sharp which links to its meaning as thin, but can mean perceptive or clever (some of us are so sharp we could just cut ourselves)
In Wordrefrence. com dictonary, Collins and else where on the web : fin is thin or fine. Google also translates this as fine. I do not see it translated as perceptive. When I look up perceptive it does not give me fin as a choice. It is percepteur, perceptif, perspicace. Is Duolingo wrong? If not show me why. Merci!
There is much more out there: try http://www.oxforddictionaries.com/translate/french-english/fin