Binnie was hesitant for courts to attempt to find the "spirit of the invention" which would create more uncertainty and unpredictability. His proposed "purposive construction" approach would avoid literal interpretation while limiting the scope of substantive claims in attempt to balance fairness between the patentee and the public. Purposive construction identifies the essential from the non-essential. The analysis is from the perspective of a " worker skilled in the art to which the patent relates as of the date the patent is published". If an essential element is different or missing from the challenged invention then there is no infringement. However, there may still be infringement where only non-essential elements are different or missing. A non-essential element is one that, from the words of the claim, are not clearly intended to be essential, or where a skilled reader would have been able to identify a substitute.