As his ''alternative'' without which opposition to the goods and services tax would be ''irresponsible,'' Jean Chretien pledges the Liberals will devise a tax system ''that will ensure . . . that businesses pay their fair share.'' Wait a minute: Isn't that what the Tories stand for? Michael Wilson himself says ''ensuring that corporations pay their fair share of tax was one of the main goals of tax reform.'' But wait a minute: Isn't that how the NDP got elected in Ontario: by promising to raise $1 billion in taxes on corporations? So we're agreed.

The proposition that the main ill afflicting Canada's tax system is corporations aren't paying their ''fair share'' has received a lot of mileage in recent years. Hence the paradox of victory in a provincial election in which the major issue was high taxes going to a party promising to raise them.

''I don't think the 'no tax' line has a lot of resonance with the public,'' Ontario NDP Leader Bob Rae said then. ''The real issue is tax fairness.'' It's not that taxes are too high, the public concluded: It's that business isn't paying.

This is the point at which apologists for business roll their eyes, and mutter ''corporations don't pay tax, people do.'' This is true, of course: All taxes on corporations are eventually passed on to people: to workers, via lower wages; to consumers, via higher prices; or to investors, via lower returns on capital. On the other hand, corporations don't avoid taxes, either: people do. If corporations are taxed too lightly relative to individuals, then there are lots of ways to transform personal income into corporate earnings. Tax specialists can do that kind of thing in their sleep.

The question is how do you measure whether corporations are being taxed too lightly. Two statistics show up over and over again in the arsenal of business bashers: the share of total tax revenues accounted for by the corporate tax, and the number of profitable businesses paying no tax in a given year. No less a figure than that master of numerology, Mel Hurtig, appeared before the Senate banking committee this summer to complain of the ''enormous tax concessions for giant corporations'' that have left many ''huge, very profitable and powerful'' companies paying little tax. The most dramatic evidence of this: ''In 1950, individuals and corporations shared about 50% of the tax load.'' But ''by 1989, individuals were carrying 88.1% and corporate income tax was 11.9% of the total tax collected.''

Correction: individuals paid 100%, which means this statistic, by itself, is meaningless. It's not the share of the total paid by corporations and individuals that matters, but the rate each pays. If the corporate tax rate were raised substantially relative to personal rates, then the corporate share of total tax collected would probably fall: Corporations and investors would shift as much income as possible out of the corporate sector. But this would hardly mean that the tax system was going light on business.

Moreover, if corporate income were to shrink relative to the rest of the economy, the share of taxes paid through corporations would also decline, regardless of policy. Research by economist Alan Douglas shows the biggest contributor to the narrowing of the corporate tax base over the years has been a sharp drop in profits, from 12% of assets in 1960 to 5% in 1985.

The other statistical favorite is dug up every year out of Statistics Canada's figures on nontaxpaying corporations. Last year at this time, the NDP's Lorne Nystrom made the HD:s with the ''news'' that 93,405 profitable corporations had escaped paying tax - in 1987. This was meant to suggest a scandal, the better to bolster the NDP's campaign for a 20% minimum corporate tax. Exactly why these firms did not pay tax was left to the imagination: if not outright tax evasion, then certainly some gaping tax loopholes.

Fortunately, economists have no imagination, or the truth would perhaps never be known. University of Toronto economist Jack Mintz has delved into the figures: Of the total $22 billion in tax deductions available to corporations to reduce their taxable income in 1985, almost 50% came from losses - legitimate, economic losses - carried forward from previous years. Presumably the NDP is not proposing to tax corporations on their losses.

The rest, true, were accounted for by Hurtig's ''enormous tax concessions'' - accelerated depreciation, inventory allowances, investment tax credits, and so forth - most of which should be and have been cut back. But several points should be made, that never are, about how this backlog accumulated, what impact it has on the economy, and how we should deal with it, before we go plunging ahead with a minimum tax.