For at least the next 70 years, Deng Xiaoping recently told a group of American, European and Japanese businessmen, the People's Republic of China will remain open to foreign investment and will encourage the growth of private enterprise in China.

No one knows why Deng, the 80-year-old senior Chinese leader, picked the figure of 70 years. That's not forever, to be sure, but it's a projection into the middle of the next century, long enough for many an investment to pay off. He stressed in these conversations that "China is a good place to invest," and that "China keeps its commitments."

Former assistant secretary of state Robert Hormats, among the group that spent a week in Peking at the invitation of the Chinese government, views the 70-year commitment as Deng's way of suggesting a long-term strategy and a policy that is going to last. All in the group agreed, he said, that "Deng has both the vision and the power to make this happen."

The assembled businessmen -- many of them chief executives of their companies -- were impressed with Deng and his promises, and especially by evidence of economic progress in China in just the past couple of years. Deng said his goal is to double Chinese gross national product once, and then again, before the end of the century to about $800 per capita.

Nothing succeeds like success: The gains the Chinese have already scored in the agricultural sector by a gradual dismantling of the commune system and a shift to household production units have impressed the political structure under Deng.

Under the so-called "responsibility system" introduced by Deng, peasants in rural areas can grow crops individually, and even go into private business. In rural China, these reforms have improved the quality of life: Peasant income has risen, enabling Chinese farm families for the first time to buy television sets, clothes, washing machines and other personal items.

Now, Deng told the foreign businessmen, the same "reforms" are being extended to the urban areas. Under an Oct. 20 proclamation, factory workers will enjoy free-enterprise incentives, including the freedom to change jobs, and a wage scale keyed to the real difficulties of their jobs.

As if to emphasize the commitment to incentive measures borrowed from capitalism, a recent front-page editorial in the Communist Party newspaper, People's Daily, said that old-style Marxism is dead, and can't be expected to solve today's complicated problems.

"Marx passed away 101 years ago, his works are more than a century old. . . . Some of his ideas are not necessarily appropriate. There are many things that Marx, Engels and Lenin never experienced or had any contact with. We cannot depend on the works of Marx and Lenin to solve our modern-day problems," the editorial said.

Even though this editorial was followed a few days later by language saying that Marx's and Lenin's works could not solve "all" of today's problems, the thrust was plain enough. "They are clearly trying to find a theoretical justification for what they are doing," one Western diplomat told the Associated Press correspondent in Peking.

To be sure, Deng's boldness disturbs some Communist Party traditionalists who resist any opening to the West, and who cling to the assumed truths of pure Marxist doctrine. Moreover, the Soviet Union, obviously annoyed by growing ties between China and the West -- especially by the improving relationships between China and the United States -- has indicated it doesn't like China's departure from Marxist themes.

That's all the more reason to encourage the Deng private-enterprise initiative. Coincidentally, a presidential task force headed by Dwayne Andreas, of the Archer Daniels Midland Co., this week urged further support and encouragement of China, noting initial success of the incentives in farming areas.

The task force warned that protectionist pressures in the American textile industry might "slow down [China's] internal reliance on market forces." The Andreas group also called for increased technical assistance, especially in agriculture, through an expanded Trade and Development Program.

What does the astute Deng want from his new capitalist friends in the West? Access to high technology, to be sure. But beyond that, as conveyed to this business group, he wants investment not merely to take advantage of the profit potential in the huge consumer market in China, but investments that will help China develop its export trade.

It's a program that makes sense: By strengthening China's infrastructure and industrial power, corporate heads not only will serve their companies' narrow profitability goals, but also certify the wisdom of Deng's policies long after he's gone, and demonstrate to the Soviet Union that Marx's 19th-century dogma belongs on the ash heap.