Brms::conditional_effects plots all 2-way interactions


Recently I was plotting the conditional effects of the following model:

fit ← (cat1, cat2, cat3) ~ Var1 + Var2 + Var3 + Var1 * Var2

In order to plot the interaction, I used the following code:

conditions <- brms::make_conditions(fit, vars = "Var1")
plot(brms::conditional_effects(fit, ask = FALSE, categorical = T, conditions = conditions))

This provides the graph I was looking for but also provides a graph for the interaction between Var1 and Var3, despite this interaction term not being specified in my model.

When looking at the documentation for conditional_effects it states:
When specifying effects manually, all two-way interactions may be plotted even if not originally modeled.

My question is, where do the values (estimates, CIs, etc.) come from if these interactions were not specified in the model? Are all interactions estimated even if they are not specified? Are the values in these graphs reliable?


I think it just means that Var1 will be plotted over different values of Var3 (or vice versa) but, crucially, the slopes will be the same because there is no interaction specified in the model (the heights of the lines may change, not their slopes).

Thank you, that makes sense.