I was told that if there is a wavefunction containing a proton and a neutron that it must be anti-symmetric under exchange of a proton and a neutron. I am having trouble understanding this. The short handwavy explaination is that they are indistinguishable via the color force. What bothers me is that they are disinguishable particles. Why should there be a requirement that an exchange introduces a minus sign? Since the particles are distinguishable, why would the exchange even give a state related to the first one ... or is that a hint that this applies for specific situations only? Any help you can provide would be appreciated.