Freedom of Speech is generally regarded as a basic and indivisible human right. It is that, of course, but it is also the basis of western liberal society, and there are important rational arguments for it.
Perhaps the most coherent defense of Freedom of Speech is John Stuart Mill's On Liberty, which is based on Utilitarianism, the idea that good is whatever makes the most people happy. His thesis is that a developing society that promotes discussion and aims for the truth brings about the best conditions for happiness.
In the end of Chapter 21, Mill details four grounds for the importance of Freedom of Speech.
We have now recognised the necessity to the mental well-being of mankind (on which all their other well-being depends) of freedom of opinion, and freedom of the expression of opinion, on four distinct grounds; which we will now briefly recapitulate.
First, if any opinion is compelled to silence, that opinion may, for aught we can certainly know, be true. To deny this is to assume our own infallibility.
Secondly, though the silenced opinion be an error, it may, and very commonly does, contain a portion of truth; and since the general or prevailing opinion on any subject is rarely or never the whole truth, it is only by the collision of adverse opinions, that the remainder of the truth has any chance of being supplied.
Thirdly, even if the received opinion be not only true, but the whole truth; unless it is suffered to be, and actually is, vigorously and earnestly contested, it will, by most of those who receive it, be held in the manner of a prejudice, with little comprehension or feeling of its rational grounds. And not only this, but, fourthly, the meaning of the doctrine itself will be in danger of being lost, or enfeebled, and deprived of its vital effect on the character and conduct: the dogma becoming a mere formal profession, inefficacious for good, but cumbering the ground, and preventing the growth of any real and heartfelt conviction, from reason or personal experience.
In other words...
- Censorship of ideas is an assumption of infallibility, as the ideas could turn out to be true.
- Discussing ideas is a process that brings us closer to truth, which is why weighing adverse opinions is valuable. Even false opinions can hold new knowledge.
- Discussion is necessary for people to truly understand the rational grounds for their own thinking and to prevent prejudiced views.
- Having an open discussion prevents truth from becoming empty dogma, harmful for human development and experience
The four grounds show why Freedom of Speech is so important for societal development and well-being. Pursuit of truth is an important quality of a functioning society, and it cannot exist without Freedom of Speech.