  Procedural planning, which entails decomposing a high-level goal into a
sequence of temporally ordered steps, is an important yet intricate task for
machines. It involves integrating common-sense knowledge to reason about
complex contextualized situations that are often counterfactual, e.g.
"scheduling a doctor's appointment without a phone". While current approaches
show encouraging results using large language models (LLMs), they are hindered
by drawbacks such as costly API calls and reproducibility issues. In this
paper, we advocate planning using smaller language models. We present PlaSma, a
novel two-pronged approach to endow small language models with procedural
knowledge and (counterfactual) planning capabilities. More concretely, we
develop symbolic procedural knowledge distillation to enhance the implicit
knowledge in small language models and an inference-time algorithm to
facilitate more structured and accurate reasoning. In addition, we introduce a
novel task, Counterfactual Planning, that requires a revision of a plan to cope
with a counterfactual situation. In both the original and counterfactual
setting, we show that orders-of-magnitude smaller models (770M-11B parameters)
can compete and often surpass their larger teacher models' capabilities.
