The research subject of this Portfolio of Essays is my ‘apparatus of thought’ in the context of my professional development. I have examined theory and the application of theory to a professional life. I analyse how my use of theory has changed and enabled my professional development. Each of the three Essays takes a different perspective and applies the theories of adult mental development, business and firm growth as an analytical framework. Using Kegan’s theories of Adult Mental Development as an ‘apparatus of thought’, Essay One is my reflection on my professional development in the light of my training, education, and development. I describe how changes in my ‘apparatus of thought’, my meaning-making system, have enabled me to develop my professional practice. Exposure to the theories of Adult Mental Development have also enabled me to understand how my development has precipitated and necessitated my career changes. In Essay Two, moving from military aviation to book publishing was a major career change for me, enabled by a change in my meaning-making system. In the context of my professional development, I sought to change my practice so that I could make a more meaningful contribution to the firm. To achieve this, I directed my reading towards a deeper understanding of the nature of the firm and the impact of industrial change on the firm. Using the context of my professional environment in Essay Three, I show how my use of theory has developed. I describe how I sought to change working practices in the firm and discuss the impact this initiative had on my professional self. I use the ‘data of my experience’ to examine my theory of the business from a Penrosian perspective. The Penrosian perspective coupled with my exposure to theories of Adult Mental Development and the effects of a transformational education process enabled me to transition to a leadership role with an international online publisher