We think about thousands and thousands of things. We imagine a horse, a man, an aero plane, the earth, a train and a book. We see the pictures of these things displayed on the screen of our imagination.
This is called the existence in imagination (Wujud-e-Dhehni).
And also a horse, a man, an aero plane, the earth or a book has its own existence outside our imagination. That is called existence outside imagination. This is the real existence (Wujud-e-Khariji).
Sometimes, we imagine such ideas, which can never be found, outside our imagination. We may imagine "2+2=5" but can 2+2 be 5 in real existence? No. We may imagine that a thing exists and also does not exist at the same place at the same time. But can this happen in the world of reality? Certainly NOT.
Such imagined ideas, which can never exist in reality, are called (Mumtaniul-wujud) 'impossible'.
Also we imagine a man walking at certain time. Can this happen in reality? Remove all other ideas from your mind. Just look at the imagined picture of that man walking at a particular time. Now say, is it necessary that the man should be walking at that time? Or, on the other side is it impossible of him to be walking at that time? The answer to both questions is 'No'. Why? Because it is neither essential nor impossible for any man to walk at a given time. He may be walking; he may not be walking. So far as the reason and logic is concerned both his walking and not walking are possible - possible but not necessary.
Such imagined ideas, which have equal relation with existence and non-existence, are called (Mumkinul-Wujud) Possible or Transient. They may exist in reality; they may not exist. There is nothing in their nature to demand this or that. So for as their nature is concerned, 'To be' and 'Not to be' both are equal to them.
So far we have seen two categories of relationship between an imagined idea and its existence in reality.
(1) Where that idea has equal relation with existence and non-existence. It may exist; it may not exist. There is nothing in its nature to prefer either side. (2) Where that idea can have absolutely no relation with existence. It, by it very nature, is non-existent.
It will appear from above classification that there should be a third category which would be opposite of 'Impossible' (Mumtaniul-Wujud) mentioned in (2) above. This third category is of the idea, which can have absolutely no relation with non-existence. By its very definition, it is self-existent. Such an idea is called (Wajibul-Wujud) 'Essential Existence' or 'Absolute Existence.'
Now the picture is complete.