Thought is an expression of the essence of the soul
ב''ה Even though "machshava" - thought is in general expressed as a lower garment of the soul the truth is that it is actually the ability of the soul that has the closest expression of the very essence. This is why it is only thought that is in a constant flow, unlike all the other abilities and all ideas can only be formed and express using through it. In some ways it is the their starting point. In this context the tought is the starting point where the essence of the soul expresses itself into the formation of ideas. This is why it says ’הרהורי עבירה קשים מעבירה’ - thoughts of sins are 'harder' than sin itself. It does not say that they are 'חמור מעבירה' - more severe in stringency that sin, rather, that they are 'harder' - meaning 'harder' on the soul since it affects and touches a much deeper part. It is because of this fact, that thought touches the essence of the soul that thoughts of sin affect us so deeply. -Mittler Rebbe