Solitude: Four Ways of Being Alone inaugurates the Themes strand of the Interactive Reading Guides — companions that follow a single idea across several thinkers at once. This guide reads solitude through four writers who understood it very differently — Michel de Montaigne, Albert Camus, Simone Weil, and Fernando Pessoa — arranged not as a list but as a spectrum, from the self that solitude fortifies to the self that solitude dissolves. Montaigne's serene, well-furnished retreat gives way to Camus's lucid revolt, then to Weil's ascetic self-emptying, and finally to Pessoa's dispersal of the self into heteronyms and saudade. Read from first to last, the self grows steadily thinner — cultivated, then defiant, then emptied, then dispersed — as active, principled solitude passes by degrees into passive, aestheticized dissolution, with Weil as the hinge where the will to solitude tips into the surrender of the self. A second thread, nostalgia, runs throughout, from a faint note in Montaigne to the whole substance of Pessoa's saudade. A comparative section sets the four side by side across shared dimensions, and a closing hands the reader the question the spectrum raises: which solitude is yours? Part of the Interactive Reading Guides series, it is meant to be read alongside the original works, which it names but never reproduces: a companion to your own encounter with them, and all commentary is original.