Logic is the study of arguments. An argument is a body of premises martialled in support of a conclusion. In Descartes' famous argument "I think; therefore I am", for example, he martialled the premise "I think" in order to support the conclusion "I am". A valid argument is an argument such that necessarily if the premises are true, then the conclusion is true as well. Descartes' argument is valid because necessarily if it is true that he thinks, then it is also true that he exists. In this course we learn some formal techniques for assessing validity and apply them to some philosophical arguments.