There seems to be an overlap in the definitions of Butterflies and Moths. I have read that Moths tend to have more elaborate antennae, and tend to rest with their wings down.
On the basis of the second characteristic, I would regard this handsome fellow as a Moth.
In fact, as far as I can tell from looking a a number of sites on the internet, it may be a Moth of the Order Macroglassinae, and named Daphnis Hypothous.
P'Dom found it in the garden at lunch-time, and brought it back to the apartment to show me. After subjecting it the the enquiring eye of my colleague Canon for a few minutes, I returned it to the garden where it blended into the greenery with great ease.
Here is another view:
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