While intellectuals engaged in public advocacy long before the term 'public intellectual' was coined, it was largely Emile Zola's cry 'J'accuse' during the Dreyfus Affair in late nineteenth-century France that gave rise to the expectation that intellectuals 'speak truth to power '. Yet, while many twentieth- and twentyfirst-century intellectuals have spoken to power either as critics or as 'fellow travellers', their public engagement has always been accompanied by the question of legitimacy: why should their opinions be valued more than those of coachmen, shoemakers or, for that matter, Facebook users? The intention in this article is to partly address this question by investigating the strategies of legitimisation and validation used by public intellectuals in their political argumentation. Focusing on one case study - the long, burdened and erratic relationship between Israel's writers and scholars, and the country's prime ministers - I propose three main sources of validation used by public intellectuals: their preoccupation with ideas, their historical knowledge and their reputation. I illustrate these three modes of validation by analysing open letters written by theologian Martin Buber, philosopher Nathan Rotenstreich, historian Jacob Talmon, novelists Amos Oz and David Grossman, and others to Israel's prime ministers from 1948 to the present, showing how the three modes evolved in response to the respective prime ministers' attitudes towards the political involvement of intellectuals and how they were combined by public intellectuals in need of effective strategies to legitimise their stand in given political situations. I then try to assess the effectiveness of such strategies and conclude by noting the challenges posed to public intellectuals today by new players in the market of ideas, especially bloggers using new sources of validation, such as their closeness to the grassroots, in their political argumentation.