Ascientific hypothesis can be valid even if it cannot be tested by observation or experimentation.
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A scientific hypothesis can certainly be valid even if it cannot be tested by observation or experimentation.  

But you don't know whether or not it's valid.

myth. it was once thought that animals, including cats and dogs, could only see in black and white. however, scientists have proven this to be a myth. in all animals, including humans, the perception of colour is determined by the presence of cells in the eye called cone photoreceptors.