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consider the following argument, this might steer your mind to some conclusion.

Suppose that It is not impossible for something to come into being without a cause. Then, consequently, if it is not impossible then it is possible for something to come into being without a cause. Now, if it is possible for something to come into being without a cause, then there is a World W1 such that in it there are event that come into being without a cause. Consequently, if there is a world in which events come into being without a cause, then it is not impossible for something to come into being without a cause. Similarly, if it is not impossible for something to come into existence without a cause, then it is possible for something to come into existence without a cause in W1. Therefore, there is a W2 accessible to W1 such that in it thing come into existence without a cause. Following the same reasoning one can deduce our world is a world such that in which things come into being without a cause. However, that is not what we observe. Therefore, by the negation of consequent it is not the case that it is possible for things to come into being without a cause.

The above aregument I gave is modally valid. That is, it is a valid modal argument. Since developement of Modal Logic is a recent phenomenon, Russell's modal conception was not as sophisticated to realise the deductive impossibility of the notion that things can come into being without a cause. That is why, perhaps, he followed Hume and threw the bathwater with the baby. Though, I have immence respect for Russell, especially for his contributions to analytic philosophy, he is not beyond reproach.