Grey Fox, Metal Gear Cosplay Costume

The overarching theme of the Metal Gear Solid series is that of the "gene, meme, scene and sense" and how people are affected by these factors according to the game's producer Kojima — Metal Gear Solid deals with genetics and the moral implications of genetic engineering, Metal Gear Solid 2: Sons of Liberty deals with how identity can be affected by the philosophies of one's society (a 'meme') and the effects of censorship on society, Metal Gear Solid 3: Snake Eater deals with how the time and place one lives in (a 'scene') affects their identity and how politics change along with the times, and Metal Gear Solid 4: Guns of the Patriots deals with the 'sense' that people die, things move on and times change and that life shouldn't be lived fighting.
The longest running theme of the series is the continued manipulation of soldiers by politicians, against which the revolutionary "Outer Heaven" ideal serves as a countermeasure. The original main villain, Big Boss, attempted to establish a purely military nation run by mercenaries solely for mercenaries. The succeeding games' villains felt they shared this ideal, coming up with new ways to create this so-called country without politics. Recently, the series has shown Big Boss in prequel games as the protagonist: a mercenary that is continually manipulated and forced through subsequent tragedies.
The games carry many implicit parallels to Nietzschean philosophy. Many of the characters struggle to find their own way of life after having discovered that the objective standards by which they lived by turned out to be a lie, a concept similar to the death of God that Nietzsche wrote about in Thus Spoke Zarathustra. The reiteration of settings, themes and plot development across different Metal Gear Solid games is a striking illustration of the Eternal recurrence, as described in The Gay Science.
"Outer Heaven" has been attempted in many forms. Big Boss attempted to build his ideal state in Southern Africa in Metal Gear and in Central Asia in Metal Gear 2: Solid Snake. Liquid Snake spoke of turning a remote Aleutian island into a sovereign mercenary state in Metal Gear Solid. Solidus attempted to free Manhattan from "the Patriots" in Metal Gear Solid 2: Sons of Liberty. In Metal Gear Solid 4: Guns of the Patriots, "Outer Heaven" is a parent company for private military companies that employs thousands of soldiers without a country.