The principles underlying the rules for brushing and combing hair on Shabbat have enormous impact on a wide range of other halachic issues.
Brushing hair is prohibited on Shabbat if hair is inevitably uprooted. This is the rule of pesik reisha.
But how do we determine whether something is inevitable? For example, switching on an electric kettle to boil water is prohibited on Shabbat because the action (switching the kettle on) is said to have the inevitable result of boiling water.
Without the power of prophecy, how are we to know whether the water will inevitably boil? There could be a power failure; the kettle could be broken; defining inevitability is therefore of fundamental importance in brushing hair and many other issues.
Primary sources are available through the links below in both WORD format and pdf. In class we will translate and explain these sources.