Proposition 3. (Itˆo’s lemma) For a given Itˆo processdSt = a(t, St)dt + b(t, St)dWt,the “stochastic” differential of a real-valued function X(t, St) (which is at least one timedifferentiable in t and twice in St) isdXt =( ∂Xt∂t + ∂Xt∂Sta(t, St) + 12∂2Xt∂S2tb(t, St)2)dt + ∂Xt∂Stb(t, St)dW
Not a mistake, I'm just struggling to understand because it feels like by turning (dWt)^2 into dt, (I know it's because of var) it feels like we are "losing randomness", I must be missing some fundamental understanding