Well-known for his work in philosophy of logic and mathematics, in both the analytic and phenomenological traditions, he was at the time of his death Professor of Philosophy at San Jose State University in California. I will add links to memorial notices as they appear.
(Thanks to Charles Carlini for the information.)
UPDATE: Philosopher Charles Parsons (Harvard) kindly writes with more information about his former student, Prof Tieszen:
Tieszen was born in 1951 and received a B. A. from Colorado State University, I believe in 1975, and an M. A. from the New School for Social Research in 1978, where he studied with J. N. Mohanty. Thus he had a background in Husserl and phenomenology when he entered the graduate program at Columbia University, where he received the Ph. D.in 1987. I was the main director of his dissertation. This led to a book, Mathematical Intuition: Phenomenology and Mathematical Knowledge (Dordrecht: Kluwer, 1989). He was a productive writer in the philosophy of logic and mathematics; he published a collection of his essays, Phenomenology, Logic, and the Philosophy of Mathematics (Cambridge University Press, 2005). In late years he turned to writing on Gödel and published After Gödel: Platonism and Naturalism in Mathematics and Logic (Oxford University Press, 2011). Before his death he completed a book on Gödel for a general audience, Simply Gödel [which will be published Mr. Carlini's publishing house in New York, Simply Charly].
After a couple of temporary jobs, Tieszen joined the faculty of San Jose State and remained there until his death. He and I kept in touch throughout his career.
He is survived by his wife, Nancy, sisters Patricia Anderson, Pamela Hobbs, Sandra Tait and parents Beverly and James Tieszen.