Since the dawn of intelligent life, humanity has searched for patterns in everything imaginable. Intelligent beings have a tendency to organize events into ideas that we can understand. We search for correlations, and ways to tie seemingly different ideas together. One way to organize data and represent it in a clearly understandable way is through numbers. Arithmancy is a branch of magical knowledge that seeks to do just that. To be clear, this subject is NOT the same as muggle arithmancy. Muggle arithmancy is a hocus pocus way of characterizing people based on the letters in their names. It is very random, useless and mindless entertainment that, much like muggle astrology, could apply to almost anyone. Arithmancy in the wizarding sense involves complex relationships between numbers to discover powerful combinations magically and predict the likelihood of large-scale events based on objective mathematical laws. This book will cover a basic first year course in arithmancy. We will discuss the properties of different numbers and why those properties exist. We will discuss numbers in relation to quantity of different items, the numeric properties of those objects and the difficulty of casting spells on different objects.  The theorem of sevens will be used along with simple mathematical formulas to calculate spell difficulty and overall power weight. Because this is a mathematics-heavy subject, a strong background in algebraic methods and basic mathematical operations is recommended.  A background in basic chemistry, potions and herbology will be helpful in understanding this subject but is not necessary.