When discerning or at least coming up with a good possibility of the true nature of existence, the best starting point is yourself. That is, look at yourself and what you truly are, disposing (for the moment) of all your conditioned or accepted ideas about the nature of existence and your guesses, let’s face it, regarding the nature of the external world.
When you do that, existence immediately appears and presents itself in the form of you—a “first person POV camera” that is, right now, experiencing something in sensory, thought, or emotional form.
That’s your starting point, your true “square one” which you can always jump back to when things get complicated, or when you start to accept the existence, and treat it as if it is as real as you, of something or things not composed of the same thing you are: subjective experience.
Begin, then, with the simple observation that you are an experience. From this “Existence 101” observation you can probably deduce, using yourself and the substance of which you can immediately see you are composed (subjective experience) that the experience that is you and that forms you had to derive from the external world, if experience is not something that did not exist before it is magically conjured by the brain (for, where is an experience before it is experienced?)
If you deny that experience is something that does not exist that is then inexplicably and magically caused to exist without use of something that already exists to create it (and magically and inexplicably stops existing when the brain ceases to function)…
…you can probably surmise experience exists in and hails from the external world, and in the absence of the ability of non-existent things to come into existence and vice versa, is probably (in terms of experience qua experience as opposed to types of experience) eternal.
Phenomenal Graffiti
Pantheopsychic Theist and Philosopher