This short Twitter post offers links to three OER texts on proof. As Richard Hammack says in the introduction to his text, "This book will initiate you into an esoteric world. You will learn and apply the methods of thought that mathematicians use to verify theorems, explore mathematical truth and create new mathematical theories." I had some exposure to this world while studying the philosophy of mathematics (and Philip Kitcher remains a fave). This link will probably interest nobody (readers will probably already know this stuff, or won't be interested, though maybe educators might want to look at the different presentation styles) but there's no way I can leave these three items unlinked. Because there's always someone who says "learning should start with the foundations" and I want to raise my eyebrow, say "really?" and ask them "which treatment of set theory we should start with then?"