We consider two different approaches for assigning force field parameters to a molecule: direct and indirect chemical perception. Indirect chemical perception is exemplified by the traditional approach to parameter assignment (such as the AMBER and CHARMM force field families) in which, once a molecule's atoms have been typed, all other information about chemical environments (including bond orders) is discarded. All force field parameters, including valence terms, are then assigned using only the atom types and the way they are connected. Thus, in indirect chemical perception, atom types encode the information needed to assign all valence and bonded parameters. Alternatively, in direct chemical perception, parameters are assigned based on the full molecular graph-a full valence representation of the molecule including elements, connectivity and bond orders, rather than just atom types and local connectivity. This approach thus involves direct analysis of the full molecular graph, including bond orders, instead of indirect analysis through connected atom types, as in indirect chemical perception. As previously detailed, direct chemical perception allows force fields to be fully specified with far fewer numerical parameters than required with indirect chemical perception. We recently introduced a new force field format, the SMIRKS Native Open Force Field (SMIRNOFF) which uses SMIRKS patterns to allow direct assignment of force field parameters, thereby implementing direct chemical perception. This is a substantial break from indirect chemical perception which uses a graph labeled with atom types to assign parameters. SMIRNOFF instead uses direct chemical perception, using substructure searches via different SMIRKS strings to assign parameters when the target molecular substructures are encountered. Thus, the chemical perception and parameters for each force term are separate from those applied to other force terms. For example, a new set of Lennard-Jones parameters could be introduced without needing to introduce additional valence terms, or vise versa.