Final answer:
The second formulation of the categorical imperative follows from the first because both are based on the principle of universalizability. The universal law formulation prohibits actions that can't be universalized, like lying, while the humanity formulation prohibits using individuals merely as means to an end, reinforcing the idea of respect for all individuals.
Step-by-step explanation:
The second formulation of the categorical imperative, also known as the humanity formulation, follows from the first formulation, the universal law formulation, because both express the underlying principle of universalizability in morality. According to the first formulation, one should "Act only according to that maxim through which you can at the same time will that it become a universal law." Following this, lying to a bookseller about being a student for personal gain would be disallowed as it could not become a universal law without defeating the purpose of lying - if everyone lied, no one would be believed.
The second formulation extends this by emphasizing that all individuals are ends in themselves and should never be used merely as means to an end. Treating individuals with the respect and consideration you would wish upon yourself aligns with the idea of universalizability. Hence, if an action cannot be universally applied or if it treats others purely as means, it both fails to comply with the universal law and disrespects the humanity of those involved, bridging the two formulations.
Immanuel Kant's work on deontological ethics emphasizes our moral duty to follow these categorical imperatives, which are derived by reason and not contingent upon personal desires or inclinations, thereby upholding the fundamental principles of moral law and rational conduct.