We live in (or at least‚ perceive‚ with various branches of string theory including up to 26 dimensions) a universe with three dimensions (plus time)‚ and there are reasons to suspect why we find ourselves in a 3D world. Scientists and mathematicians have suggested universes with more than three dimensions would be unpredictable and unstable &;quot;dead worlds&;quot; devoid of all life and observers. The three body problem is unpredictable in a 3D world‚ but even a two body problem – describing and predicting the orbit of two bodies – becomes too chaotic in higher dimensions‚ and no stable orbits are possible. &;quot;This means that such a world cannot contain any objects that are stable over time‚&;quot; one paper on the topic explains‚ &;quot;and thus probably cannot contain stable observers&;quot;.&;quot;In a space with more than three dimensions‚ there can be no traditional atoms and perhaps no stable structures‚&;quot; the paper adds.As such‚ we should not be surprised that we find ourselves in a 3D (plus time) universe – you can only live in universes where life is possible. There are suggestions that life could not take place in a 2D (plus time) universe‚ meanwhile‚ because of insufficient complexity. The main argument‚ which would make 2D universes a non-starter‚ is that two-dimensional universes would not allow for gravity‚ making the complex mix required for life impossible.But that may not be the case‚ according to physicist James Scargill. In a 2020 paper‚ Scargill showed that scalar gravitational fields could exist in two dimensions.     &;quot;I have presented a purely scalar theory of gravity which allows stable orbits around point sources‚ and has a not-obviously-fatal (though unusual) cosmology; it could potentially be improved by making the whole metric dynamical‚&;quot; Scargill writes in the paper. &;quot;One could also imagine a brane-world scenario in which the massless graviton is not localized to the brane‚ thus allowing two-dimensional life to enjoy fully four-dimensional gravity.&;quot;Gravity and stable orbits are not the only requirements for life – or‚ more important when you are using anthropic reasoning to explain why we are in a 3D universe‚ for observers to exist. For instance‚ you could not have a digestive tract in an animal (at least not the kind we are used to) as it would split the organism in two. Scargill went on to look at whether a 2D universe (again‚ plus time) would be sufficiently complex to allow for complex life. In the paper‚ he looked at biological networks and created planar graphs which &;quot;seem to exhibit many of the properties which have been conjectured to be important for complex brains.&;quot; This‚ he writes‚ is suggestive that complex brains could exist in two dimensions‚ though more work would be needed to compare the planar graphs to real-world neural networks.&;quot;In particular‚ they are approximately ‘small-world‚’&;quot; he added‚ &;quot;they have a hierarchical and modular construction‚ and they show evidence of the stretching (in parameter space) of a critical point into a finite critical region for certain stochastic processes.&;quot;This is‚ of course‚ highly hypothetical‚ and more of a thought exercise than saying life in 2D universes is real. But it puts a bit of a constraint on arguments that we experience a 3D world because it is the only type that could sustain life‚ because – perhaps – life in 2D could be possible as well.The paper is published in Physical Research Review.