Can we find a non-cartesian monoidal category where for each pair of objects the tensor product $A \otimes B$ is a cartesian product of $A$ and $B$? Let me explain. A [monoidal category](https://en.wikipedia.org/wiki/Monoidal_category#Formal_definition) $(\mathsf{C}, \otimes, I, a, r, \ell)$ is a category $\mathsf{C}$ with a tensor product $\otimes$, a unit object $I$, an associator $a$, and left and right unitors $r$ and $\ell$ obeying [the pentagon and triangle identities](https://math.ucr.edu/home/baez/qg-winter2001/definitions.pdf). A cartesian category is one with [binary products](https://en.wikipedia.org/wiki/Product_(category_theory)#Definition) and a [terminal object](https://en.wikipedia.org/wiki/Initial_and_terminal_objects). Given a cartesian category $\mathsf{C}$, and choosing a terminal object, and choosing a product cone $$ A \leftarrow A \times B \rightarrow B$$ for each pair of objects $A,B$, there is a well-known way to make $\mathsf{C}$ into a monoidal category. To create this monoidal category, first we take $A \otimes B$ to be the object $A \times B$ in the chosen product cone for $A$ and $B$. Then we define $\otimes$ on morphisms using the universal property of the chosen product cones. Similarly, we define the associator $$ a_{A,B,C} \colon (A \otimes B) \otimes C \to A \otimes (B \otimes C)$$ using the universal property of the chosen product cones. Then we take the unit object $I$ to be the chosen terminal object, and define the left and right unitors $$ \ell_A \colon I \otimes A \to A, \qquad r_A \colon A \otimes I \to A$$ using the universal property of the chosen product cones and the fact that $I$ is terminal. We obtain a monoidal category $(\mathsf{C}, \otimes, I, a, r, \ell)$ called a 'cartesian monoidal category'. More generally, any monoidal category *equivalent as a monoidal category* to one of this form is called a **[cartesian monoidal category](https://ncatlab.org/nlab/show/cartesian+monoidal+category)**. This leaves open the possibility that there could be a way to make our cartesian category $\mathsf{C}$ into a monoidal category where for each pair $A, B \in \mathsf{C}$ the object $A \otimes B$ equal to the chosen object $A \times B$, but which however is *not equivalent*, as a monoidal category, to the cartesian monoidal category described above. **Is this ever possible?** It would be especially fun if we could do this starting from some familiar cartesian category like the category of sets.