This lecture introduces the concept of systems thinking as a powerful tool to understand the different levels of complexity in the world, using qualitative and quantitative approaches. Through a case study on forest management in Canada, the instructor illustrates how systems dynamics, such as stocks, flows, feedback loops, and positive and negative reinforcement loops, play a crucial role in ecosystem balance and resilience. The lecture also delves into the ecological and economic implications of human interventions in natural systems, emphasizing the importance of questioning the need for constant resource extraction and exploring alternative societal and economic models that align with natural cycles.