Two logical sentences are central to quantified modal logic – the Barcan and Converse Barcan Sentences. Here, we look at what they are, why they're philosophically contentious, and when they're valid or invalid. 00:00 - Intro 01:25 - Ruth Barcan Marcus 02:25 - Barcan Sentence 02:52 - Converse Barcan Sentence 03:47 - Are they true? 04:01 - Exercise 04:22 - Worked Answer 05:30 - Why it’s controversial 07:19 - When is BS valid? 10:06 - Nested domains model 10:43 - Shrinking domains model 11:12 - When is CBS valid? 11:30 - Summing up More videos on modal logic coming next! If there’s a topic you’d like to see covered, leave me a comment below. Links: My academic philosophy page: http://markjago.net My book What Truth Is: http://bit.ly/JagoTruth Most of my publications are available freely here: https://philpapers.org/s/Mark%20Jago Get in touch on Social media! Instagram: https://www.instagram.com/AtticPhilosophy Twitter: https://twitter.com/PhilosophyAttic #logic #philosophy #possibility