Final answer:
The phrase in question is Kant's humanity formulation of the categorical imperative, which asserts that one should act in a way that treats humanity as an end in itself, never as a means only.
Step-by-step explanation:
The phrase '"so act as to treat humanity, whether in thine own person or in that of any other, in every case as an end withal, never as a means only" is Immanuel Kant's version of the categorical imperative specifically related to the principle of humanity. Kant's philosophy states that we must always treat others as ends in themselves, never simply as a means to achieve our own goals. This formulation is known as the humanity formulation of the categorical imperative, which along with the universal law formulation serves as a basis for Kant's ethical theory.
For Kant, moral actions are those that respect the intrinsic value of humans as rational beings and do not use them merely as tools for personal gain. Hence, this imperative goes beyond the guidance of the hypothetical imperative, which only focuses on actions taken for certain outcomes or goals. Consequently, when relying on human research subjects, they must be treated as ends in their own right, ensuring that their rights and autonomy are fully respected and that their participation is based on an informed and voluntary decision.