If you wish to make an imaginary animal invented by you appear natural, let us say a dragon, take for the head that of a mastiff or hound, for the ey… - Leonardo da Vinci

Fire destroys all sophistry, that is deceit; and maintains truth alone, that is gold.

When you represent in your work shadows which you can only discern with difficulty, and of which you cannot distinguish the edges so that you apprehend them confusedly, you must not make them sharp or definite lest your work should have a wooden effect.

If you take as your pattern the wings of feathered birds, these are more powerful in structure of bone and sinew because they are penetrable, that is to say the feathers are separated from one another and the air passes through them. But the bat is aided by its membrane, which binds the whole together and is not penetrated by the air.