Is classifier able to say there's no-such-case? I am a starter in ML and I need some help...
The problem
Assume that I have a classifier which can classify left hand / right hand well.
I am curious whether it can decide whether there's a hand in the image?
Because if it is not the case, I have to collect additional data that are labeled as &quot;no-hand&quot;. But there's so many things that are &quot;no-hand&quot; such like dogs, cats, flies... I don't think it is possible to collect all those possible &quot;no-hand&quot; data..
My intuition
Would it be reasonable that we use some activation functions like [sigmoid] as the final outputs for the classifier(neural network) and we consider the outputs for both left/right classes as the scores that classifier think how far the input is from the left/right hands classes.
Then if the outputs are below some threshold for both left hand and right hand we can say there's no hand in this image input.
Taking your questions one after the other:


  Assume that I have a classifier which can classify left hand / right
  hand well. I am curious whether it can decide whether there's a hand
  in the image?


No. It cannot. A model which is trained on hand/no-hand can do that. Not this. At most, it shall predict one of the either (left/hand) with a very low confidence score. But, it definitely is not a nice idea to use it for a hand/no-hand identification.


  But there's so many things that are "no-hand" such like dogs, cats,
  flies... I don't think it is possible to collect all those possible
  "no-hand" data


You are trying to do build a binary hand.no-hand classification model, not a hand/cat/dog/... style multi-label classification model. So, you just need to have a bunch of images with a hand in it, and some more which doesn't have one.


  Would it be reasonable that we use some activation functions like
  [sigmoid] as the final outputs for the classifier(neural network) and
  we consider the outputs for both left/right classes as the scores that
  classifier think how far the input is from the left/right hands
  classes.


Possible, but not advisable.