First, a disclosure: Shashi Tharoor is an old friend and colleague. I edited his column, "Worldview", in one of my magazines for over ten years. His twin sons and my son were born in the same week, month and year and often lay in cradles side-by-side as we spoke on politics and life over dinner at each other's homes.

That said, Shashi's columns in my magazine received a more animated response than any of the other columns we carried at that time: Dr Henry Kissinger, Dom Moraes, LK Advani, Madhu Dandavate, IK Gujral and several others.

Shashi's 15-minute speech at the Oxford Union recently, as part of an eight-speaker debate on whether Britain owes its ex-colonies financial reparations, has gone viral with over a million views on YouTube. And so it should.

Listen to the points Shashi makes in the video. Each one is valid - though he does commit two small errors.

First, Britain's "aid" to India is not 0.4 per cent of our GDP as he says. It's one-twentieth of that - 0.02 per cent ($410 million on a GDP of $2.10 trillion). Much of this aid, moreover, is tied to buying British goods and services. A better way to describe it would be "trade aid".

The second error lies in Shashi's concluding remarks at the Oxford Union debate: He says reparations for Britain's colonial debt are theoretical since they can't be quantified. An apology from Britain will therefore do.

He's wrong on both counts. Britain's colonial debt to India can in fact be quantified. And an apology alone won't do.

In an article I wrote for The Illustrated Weekly of India over 25 years ago - then edited by Pritish Nandy - I did precisely that: quantify Britain's colonial debt to India. Most of my article focused on the same areas as Shashi's Oxford speech: Britain's colonial rule destroyed India's industries and made India a vassal for the newly industrialising Britain in the 19th century. But of that computation in a moment.

In yet another article for a daily newspaper, in 2007, I looked at Britain's global colonial debt in historical terms:

"One of the great hidden secrets of British colonialism is the slave trade. It was a 250-year-long trade that disgraced Britain and brutalised an entire continent. British historians routinely gloss over it and Africa, hit twice by European rapacity (first by the slave trade, then by the colonial carve-up of the continent in the 19th century once "slaving" had officially ended), has not yet developed the historical scholarship to deal with and expose the full horrors of slave trafficking."

"Lord Hugh Thomas, one of the world's most accomplished historians on the Atlantic slave trade, describes the hellish journey inland even before the slaves were packed on to the slave ships: 'The slaves were usually secured by placing the right leg of one and the left leg of another onto the same pair of fetters. If the fetters were connected by a string, these men could walk, though slowly. Every four slaves might also be fastened together by the necks, with a strong rope of twisted thongs and, at night, additional fetters would to put on their hands. Sometimes, a chain would be passed round their necks. Those slaves who protested were imprisoned in a thick billet of wood about three-feet-long and a smooth notch being made upon one side of it. The ankle of the slave was bolted to the smooth part by means of a strong staple, one ring of which was passed on each side of the ankle. All these fetters and bolts were made from African iron."

"In 2006, Andrew Hawkins, a descendant of Sir John Hawkins - the slaving pioneer - travelled to Gambia, a west African country from where thousands of Africans had been kidnapped and sold into slavery. He went, self-bound in chains, along with other chained white Englishmen and made a symbolic apology on behalf of his family to the people of Gambia. He was received by the vice president of Gambia, Isaton Njie Saidy, and 25,000 other ordinary Gambians."

Here, in Hawkin's words, is what happened next: "I apologised on behalf of my family. I apologised for the adults and children taken. There was a long pause and we really didn't know what to expect - it was very nerve-wracking. They could have said, 'We don't accept your apology, go away,' and were ready for that. It would have been understandable. But the vice president came forward and accepted the apology very graciously. She offered her forgiveness and then came forward and took the chains off. That was entirely impromptu and very moving."

Hawkins and his volunteers apologised to the vice president of Gambia for the terrible injustice caused by Britain to Africa. But so far, no British prime minister has even hinted at a formal apology. The reason: an official government apology to Africans for the slave trade would open the floodgates for financial reparations and punitive damages for the crimes committed on the African people over a 200-year period. And those punitive damages would be substantial.

The small compensation British courts have forced the UK government to give to the victims of the Mau Mau uprising in Kenya in the 1950s, based on classified documentary evidence made public in Britain, is the thin edge of the wedge. At a notional, inflation-adjusted cost of one lakh dollars per person in terms of net lost lifelong wages in his own country, the punitive damages on Britain as the principal slaving country would amount to two trillion dollars. Such reparations could change the face of modern Africa. But they would bankrupt Britain.

So how do we quantify Britain's colonial debt to India? The computation has to take into account the following factors:

One, the difference between a) India's GDP growth rate during 1757-1947 (the 190-year period of the British occupation, first under the East India Company and, from 1858, directly under the British government), b) India's historical GDP growth rate (pre-1757) and c) its contemporary GDP growth rate (post-1947).

Two, the total value of goods and services exported during these three periods.

Three, the comparative social factors in these three periods - literacy, birth rates, death rates, infant mortality.

I computed all of these in my Illustrated Weekly piece of 1988 and, after adjusting for inflation, the debt owed by Britain to India in 1988 worked out to $500 billion. In today's (2015) money that would be closer to three trillion dollars.

As Shashi rightly points out in his Oxford Union debate, the cost of human rights abuse by the British is impossible to quantify. (The Churchill-induced Great Bengal famine of 1943 and the Jallianwala Bagh massacre of 1919 were only two of the many atrocities that are impossible to quantify in terms of financial debt.) The economic debt though can be quantified.

So will India file a case in the International Court of Justice at the Hague to press Britain for financial reparations? Unlikely. India is a forgiving nation.

Shashi Tharoor, now 59, was "scolded" by the UPA president Sonia Gandhi for not toeing the Congress line on disrupting Parliament till Sushma Swaraj, Shivraj Singh Chouhan and Vasundhara Raje Scindia resign. At a meeting of the party on Tuesday, July 21, Shashi was the only one who disagreed with Sonia's diktat: "First resignations. Then discussions."

The next day, Sonia pulled him up, saying: "You always do this. It has become a habit with you."

Perhaps Sonia should have a look at her Thiruvanthapuram MP's Oxford Union speech on European, particularly British, colonialism which was harsh, dictatorial and brooked no dissent. She surely does not want the Congress - and especially the few sensible voices left in it - to follow in such unworthy footsteps.