The proper reaction to the military reductions and withdrawals announced this week by Soviet leader Mikhail Gorbachev is to be encouraged but not enthusiastic.

Encouragement is appropriate because Gorbachev is saying all the right words, including even an admission that communist countries had under-estimated the strength of capitalism.But there's still room for some skepticism because Soviet deeds count more than Soviet words, because those deeds will be hard for the West to measure, because the timetable for the cutbacks and withdrawals is still not clear, and because the Kremlin is motivated by necessity rather than altruism.

Moreover, keep in mind that the withdrawal of Soviet troops from Eastern Europe seems bound to put pressure on the United States to pull troops out of Europe even though a mutual withdrawal would leave Russian forces much closer to Europe than American forces would be.

Anyway, Gorbachev's remarks this week simply spell out in somewhat greater detail the promise he made last month at the United Nations. Among other specifics, this week he mentioned a 14.2 percent cut in the Soviet defense budget, a 19.5 percent cut in military hardware, and a 12 percent reduction in troop strength.

The trouble with those seemingly concrete figures is that they will be hard to verify. The size and composition of the Soviet military has long been a closely guarded secret. Russia still has not published even an estimate of its total defense spending, let alone a detailed defense budget of the kind available in the United States. In recent months, Soviet officials have admitted that publicly identified defense spending accounts for only a fraction of total spending. Western estimates of Soviet military strength vary widely.

What's not a secret, however, is that the military cutbacks are being promised not because the Soviets have seen the error of their ways but because their economy has turned so sour. The worst harvest in three years has brought on nagging food shortages. Other consumer products are chronically in short supply.

But as long as the Soviet Union really shifts from an aggressive to a defensive posture, the reasons for the change and its extent are not as important as the new direction the Kremlin seems to be taking. The challenge for the Free World is to make sure the Russians keep up the momentum.