Closure has been described as an online Flash game "about being and nothingness". The game indeed takes a very original approach to the worn-out genre of platform games, taking what you see is what you get to a philosophical level.
Spoiler ahead! It's more interesting to discover the game's concept by yourself before reading on.
Lacking any instructions, Closure is the kind of game where figuring out what to do is (part of) the gameplay. In a kind of radical subjectivism, the sparsely lit black and white game world only exists insofar as it is illuminated. In other words, what you can't see isn't there, and if you tread there your character dies in the void.
The first aim of the game, therefore, is to stay in the light. Beyond this the gameplay is quite simple (getting from A to B), and the challenge lies mostly in learning how to harness the game's peculiar logic of light and dark, being and nothingness.
In philosophy, the notion of reality limited to subjective perception is rather confusingly called idealism. It is best illustrated by George Berkeley's motto, "To be is to be perceived or to perceive" ("Esse est aut percipi aut percipere"). In other words, reality doesn't exist outside our own perception. For Berkeley, the reason most of us still experience the same reality is God, who constantly labors to create a consistent world for all of us.
In Closure this stagehand God is clearly absent, and reality instantly disintegrates outside the fragile circle of light/perception. Combined with a creepy soundtrack and very, uhm, darkish graphics, it makes for a haunting, claustrophobic experience.