Photographed by Ben Richards, Swansea, Wales, 1947
- Ludwig Wittgenstein @ Encyclopedia Britannica
- Ludwig Wittgenstein @ Stanford Encyclopedia of Philosophy
- Ludwig Wittgenstein @ Internet Encyclopedia of Philosophy
- Ludwig Wittgenstein @ viaf.org/viaf/24609378/
- Ludwig Wittgenstein @ id.loc.gov/authorities/names/n79032058
- Ludwig Wittgenstein @ worldcat.org/wcidentities/lccn-n79-32058
- Ludwig Wittgenstein @ wikidata.org/wiki/Q9391
- Ludwig Wittgenstein @ d-nb.info/gnd/118634313
unnatural doubts 