  The initial mass function (IMF) of stars is a key quantity affecting almost
every field of astrophysics, yet it remains unclear what physical mechanisms
determine it. We present the first runs of the STARFORGE project, using a new
numerical framework to follow the formation of individual stars in giant
molecular clouds (GMCs) using the GIZMO code. Our suite include runs with
increasingly complex physics, starting with isothermal ideal
magnetohydrodynamic (MHD) and then adding non-isothermal thermodynamics and
protostellar outflows. We show that without protostellar outflows the resulting
stellar masses are an order of magnitude too high, similar to the result in the
base isothermal MHD run. Outflows disrupt the accretion flow around the
protostar, allowing gas to fragment and additional stars to form, thereby
lowering the mean stellar mass to a value similar to that observed. The effect
of jets upon global cloud evolution is most pronounced for lower-mass GMCs and
dense clumps, so while jets can disrupt low-mass clouds, they are unable to
regulate star formation in massive GMCs, as they would turn an order unity
fraction of the mass into stars before unbinding the cloud. Jets are also
unable to stop the runaway accretion of massive stars, which could ultimately
lead to the formation of stars with masses $\mathrm{>500\,M_\odot}$. Although
we find that the mass scale set by jets is insensitive to most cloud parameters
(i.e., surface density, virial parameter), it is strongly dependent on the
momentum loading of the jets (which is poorly constrained by observations) as
well the the temperature of the parent cloud, which predicts slightly larger
IMF variations than observed. We conclude that protostellar jets play a vital
role in setting the mass scale of stars, but additional physics are necessary
to reproduce the observed IMF.
