The Rajya Sabha today paid rich tributes to Dr. S Radhakrishnan the first Chairman of the Upper House and former President on his birth anniversary, which is also observed as Teachers Day. Making a reference in the House, Chairman Mohammed Hamid Ansari said Dr Radhakrishnan was an internationally acclaimed philosopher, an evocative teacher par-excellence and a humanist to the core. Mr Ansari said his birthday is commemorated as Teacher's Day in view of his outstanding contribution to the Indian education system. The House also expressed profound appreciation of and gratitude to the teaching fraternity for their invaluable contribution in moulding the formative years of the future citizens of the country and inculcating in them moral and ethical values.