Welcome back. We'll now begin our lectures again with Learn. We'll start by revisiting the concept of learning which we studied during the listen lecture where we learned about the naive base classifier. I will try to put that on a more firmer, formal footing as well as, Make it generalized to other forms of learning. In particular, unsupervised learning where we don't necessarily have a training set like we did earlier. We'll learn how to learn business rules or other kinds of rules from data. We'll also figure out how to learn the classes and features themselves, and study how this happens in recommender systems such as book recommendations, product recommendations, movie reviews, and stuff like that on the web. We will also speculate on how this tells us something about how human beings learn basic concepts about the world without ever being taught explicitly what features to look for. Next, we'll turn to supervised learning again where we'll figure out how to learn facts, such as, antibiotics kill bacteria, or Einstein was born in Ulm, and other things that we can learn formally from large collections of text such as that is available on the web. We'll then speculate on whether knowledge learned in this manner. Whether it's by, by crawling through large volumes of text. Or learning classes an features from data or rules. Actually, constitutes knowledge of any kind and, philosophically, does it give us some satisfaction that we've figured out something about how we all learn so much about the world. So, gets, get ready. We'll be delving deeper and covering a large tract of material this week. We'll also be doing things a bit more formally, a little more notation, some mathematical notation. But, for those of you who get confused, hopefully you won't. But, please remember that in the end, it's all about counting. So, don't get scared of the notation. Be careful to remember what it's all about. Counting, frequencies, probabilities, and essentially, Arithmetic. So, Let's get started.