These notes were prepared over a period of two years, begun at the University of New South Wales and finished at Carleton University. I am indebted to both mathematics departments for their support in many ways. As well as my colleagues in these departments I owe a special acknowledgement to L. G. Kovacs at the Australian National University; although he has not read the manuscript, he helped me at a number of crucial points with generous advice and gentle criticism. Finally, I wish to thank Gill Chater, Jan Senior and Lynn McClelland who cheerfully and competently converted my handwriting into beautiful type.