Now that President Reagan has sounded the trumpets for a charge against the Internal Revenue code, the hard work of tax simplification and reform begins. How are those of us who are not experts in the intricacies of tax law -- and don't want to become experts -- going to judge the merits and demerits of what all these different people and groups are proposing?

It helps at the start of a prolonged process like this to jot down the criteria by which you want to judge the final product. When it's over, you can look back at the list and decide how well they've done. My checklist consists of the Four Ps.

The first is Productivity. This is a practical imperative. We live in an increasingly competitive international economic environment. Our workers are competing for jobs, not just with each other, but with the workers of Japan, Western Europe and the Third World. Our firms are competing with their companies for markets and for profits.

This is a fact of life; maybe, the most important reality we face. No theoretical or ideological or emotional argument for "socking it" to this group or that can be given greater weight than that practical reality. Many reformers want to make business in general, or a particular industry, "pay up." That's fine, but not if the cost is to make them noncompetitive in the world market.

This criterion should be of help in deciding which business tax preferences are defensible and which are not. There's a difference between domestic service industries -- restaurants, for example -- and international businesses, such as steel and computers. Diners cannot easily fly to London to get a better tax break on a meal; they can do their resource buying or investing overseas. The tax code has to reflect those realities and emphasize incentives to investment over consumption.

The second criterion is Progressivity. Our income-tax system is progressive; it's based on ability to pay. The rates increase as the tax bracket rises. All the major proposals reduce the number of brackets, and that automatically reduces the progressivity of the system.

You can offset that by closing down some of the deductions or exemptions that benefit mainly upper-income taxpayers. It takes a sharp pencil to calculate who the winners and losers will be.

But the criterion has to be that progressivity is maintained in the income- tax system. That is particularly critical, as payroll taxes for Social Security continue to climb and as state and local taxes (generally less progressive than federal) rise to meet the growing responsibilities of those levels of government.

The third criterion is Principle. In tackling something as complex as the Internal Revneue code, there are constant temptations to cut deals to take care of this or that constituency. Any reform will obviously require compromise, and there is nothing dishonorable about that.

Compromise becomes dangerous when those who are not party to the deals cannot see any principle that is employed to guide the choice. Then cynicism undermines the product.

Since the whole purpose of tax reform is to restore confidence in the fairness of the tax system, visible principle has to be maintained. Are we going to give tax breaks to real estate? Then let's debate the principle of home ownership that lies behind these breaks, and see if we want to apply it to commercial buildings and to resort properties.

Are we going to repeal the deductibility of state and local property taxes? Then let's debate the principle of federalism that underlies that deductibility, and see if it's a principle we're prepared to abandon or compromise for the billions of revenues the federal government will gain.

In every case, let's put the principle front and center, and measure the tax code changes by that principle.

My final criterion would be Poverty, and while I list it last, maybe it belongs at the top of the checklist. The poor and near-poor will have fewer lobbyists and advocates outside the Ways and Means and Finance committees' doors than anyone else -- and they have a large stake in the outcome.

Just how large is demonstrated in a study last week from the Coalition on Block Grants and Human Needs called "Untax the Poor." Using data from the House Ways and Means Committee, it showed that a family of four at the poverty level paid 10.5 percent of its income in federal taxes this year -- 21/2 times as much as it did in 1978.

Many families just beyond the poverty line are being taxed back into poverty by the existing code. That is probably not something anyone intended to see happen in the last big round of tax revision, but it shows the lack of attention the poor characteristically receive in this process.

It will shame us all if it happens again.