John Stuart Mill's powerful assertion, found in his seminal work 'The Subjection of Women' (1869), encapsulates a foundational argument for gender equality. Mill, a staunch advocate for individual liberty and utilitarianism, viewed the legal and social subordination of women as not merely an injustice but a significant impediment to the progress of humanity as a whole.

When Mill states that the subordination is "wrong in itself," he appeals to fundamental principles of justice and individual rights. He argues that denying half of humanity equal opportunities, rights, and autonomy based solely on sex is an arbitrary and morally indefensible practice. It is a relic of a primitive era where might made right, rather than a system based on reason and fairness. For Mill, every individual, regardless of gender, possesses the capacity for reason and self-development, and to legally or socially restrict this capacity is a profound violation of their inherent dignity and liberty. It creates a system where one group is arbitrarily privileged over another, fostering an unjust power dynamic within society and even within the family unit.

Furthermore, Mill contends that this subordination is "one of the chief hindrances to human development." This argument is rooted in his utilitarian philosophy, which seeks to maximize overall happiness and well-being. He identifies several ways in which gender inequality stifles societal progress:

1. Loss of Talent and Contribution: By confining women to domestic roles and denying them access to education, professions, and public life, society effectively squanders the intellectual, creative, and productive potential of half its population. Imagine the scientific discoveries, artistic masterpieces, political innovations, and economic contributions that remain unrealized due to these restrictions. This represents an enormous loss to human civilization.

2. Stifled Individual Growth: Women themselves are prevented from developing their full capabilities, leading to personal unhappiness, intellectual stagnation, and a sense of unfulfillment. A society that limits the self-realization of its members cannot be truly flourishing.

3. Negative Impact on Men: Mill also argued that men are harmed by this system. They are burdened with the sole responsibility of providing, and their relationships with women are often based on dominance rather than genuine partnership and intellectual companionship. This can stunt men's moral and emotional development, fostering arrogance and a lack of empathy.

4. Societal Stagnation: A society that perpetuates such a fundamental injustice cannot achieve its full potential in terms of moral, intellectual, and social progress. It breeds prejudice, limits critical thinking, and maintains an outdated social structure that is ill-suited for modern advancement.

5. Distorted Family Life: The family, which should be a school of liberty and justice, becomes a school of despotism when one partner holds absolute power over the other. This impacts the upbringing of children, who learn about power dynamics rather than equality.

Mill's solution was radical for his time: full legal equality, equal access to education, and equal opportunities in all professions and political life. His arguments laid much of the groundwork for modern feminist thought and continue to resonate today. While legal subordination has largely been dismantled in many parts of the world, social, economic, and cultural inequalities persist. The quote remains highly relevant as societies continue to grapple with gender disparities, highlighting that true human development requires the full and equal participation of all individuals, irrespective of their sex.