Edmund Husserl called the phenomenological explorations or philosophical reflections he undertook on an almost daily basis "monological meditations." He thought with pen in hand, writing down his thoughts as they cam to him, using a stenographic notation.
Some of these reflections were short notes. Others are quite long, covering a dozen pages. But most of them cover between "three and four pages of stenographic writing," which according to Iso Kern, one of the editors of these manuscripts, represent a "day's work." Often, different texts go over the same problems. Husserl seems to have circled again and again around the same or similar problems, attempting to find a solution from different points of view. Often he just repeated himself (XIII, p. xviii). 
It appears to have been one of his maxims that one should "always go over his old manuscripts, to improve and copy them" (XXIV, p. 47).
The reason for Husserl's insistence on thinking with a pen was his belief that "the permanent fixation of acquired truth and its justification in literary form do not just make it possible for the person who discovered them to repeat the discovery with insight and happily to enjoy it again, but [they] also provide a helpful starting point [Prämisse] for the justification of new truths" (XXVI, p. 84). This does not exhaust the usefulness of "the permanent fixation of one's thoughts is not just important for the inquirer himself. One of the "important function[s] of documenting written expression" is that it overcomes subjectivity and makes the discovery intersubjectively accessible. It makes possible "communication without an immediate or mediate personal addressee, and it thus becomes a virtual communication" (VI, p. 371). This, ultimately, is how all of human knowledge progresses.
But Husserl did not think that all meditations led to the truth—at least not directly. Kern claims that what he "wrote thus in meditation was less about what he knew than about what he did not know. He did not write to record insights and ideas, but he tried gain insights in writing and thinking. In doing so, he often tried out dialectically different conceptions or possibilities of thinking, without committing to any one of them. His 'research manuscripts' offer therefore less results than possible routes, often also mistaken ones" (XIII, p. xix).
Husserl provided some of his manuscripts with marginal comments when he re-read them, like "that does not work," "bad," or "nota bene" or "to be expanded." But he also thought that any kind of written thinking involves different levels of reflection. There is, he found, an "inevitable sedimentation of the products of the mind in form of lasting linguistic acquisitions" (VI, p. 371). What might surprise some is that even Husserl's ideal objectivities are bound to their material expressions in the form of ink on paper.
One might thus argue that, for Husserl, no intellectual progress is possible without writing. This would mean that he agreed with Luhmann, who learned a lot from Husserl and said: "One cannot think without writing." Or at least one cannot write "in a precise way that makes connections possible." I would agree at least to this much.
1. All quotations are from Husserl's Gesammelte Werke. The Roman numerals indicate the volume, the Arabic numerals the page number.