To estimate the partial effect of x2 on y while x3 is kept

Question

To estimate the partial effect of X₂ on y while X₃ is kept constant, we need to estimate a regression model like y = β₁+β₂x₂+β₃x₃+u. Some students find this counter intuitive: if we're keeping x₃ constant, why should it be in our model at all? Explain intuitively why this is the right thing to do.