The Existence of Essence
It is channeling the caster’s essence that allows him to conjure items from nothing. The items are not created from thin air, they are woven from the caster’s inner essence. Should the caster grow weary, that fact will be represented in the item that is summoned. It will be a much lower quality.
The item may even dissipate into nothingness if the caster is tired or attempting to summon something beyond his skill. As a caster’s essence becomes more refined, he is able to attempt more intricate summonings. Some of the most prominent magicians have been able to summon up incredible items of power that are permanent fixtures in the world.