PERHAPS YOU feel that the country needs more vigorous speculation in commercial real estate. Perhaps, in your opinion, the national economy is being strangled by a shortage of shopping centers. In that case, you will enthusiastically support the sweeping changes that the Reagan administration proposes for the taxation of business. Otherwise, you ought to think twice.

Perhaps you think that the automobile industry is already sufficiently favored by the tax code, despite its terrific financial losses, and already provides sufficient incentives to investors. The Reagan bill would actually lengthen the present depreciation schedules for the dies, patterns and other special tools that are roughly half of an automobile company's investment flow. Of all major industries, the automobile manufacturers would be offered the least by this bill. If you doubt the wisdom of that balance, you need to take another look at this bill.

This revision of business taxation is an attempt at simplification -- radical simplification, impelled by the same radical spirit that touches evey part of the Reagan tax policy. By simplifying and sharply speeding up the depreciation of most business assets, the bill attempts to compensate investors for inflation. But there could hardly be a better illustration of the familiar principle of tax law that fairness requires complexity, and the law can be made simpler only by making it less fair.

The drastic increase in tax benefits for commercial buildings is intended to induce a wave of construction of new factories. But the administration chose not to distinguish between factories and stores. Its bill would allow the depreciation of owner-occupied stories in 10 years and rented stores in 15. There are already uneasy murmurs of protest from small businesses that typically use rented space and see their larger and richer competitors paying less for their quarters because they are able to build their own and depreciate them faster.

These extraordinary increases in depreciation benefits may well encourage more investment. But they will certainly create a two-class system of business taxation in which the heavily capitalized companies pay little tax while all the others pay much more. The administration seems to feel that all investment is equally good and equally to be spurred on. But it's not always the high-investment industries that promise the greatest progress in technology or productivity. In some of them -- steel comes to mind -- even substantial investment might bring only marginal improvement in a field that does not require expansion of production capacity.

The Reagan administration's proposed simplifications are not, in fact, so simple as they look. Congress is now about to begin rewriting the president's bill. As the debate begins, it will be useful to remember that these changes in the depression rules would affect different industires very differently. When the Reagan bill calls for five-year depreciation of machinery, that includes both the auto dies now depreciated in three years and the oil refineries now depreciated in 16 years. The eccentric distribution of these benefits would powerfully affect the terms on which companies and industries compete with each other. There is no evidence whatever, in its program or in its testimony, that the administration has thought much about these consequences.That's a good reason for Congress to think about them very carefully indeed.