EXHIBIT 5.1

                     [Letterhead of Pillsbury Winthrop LLP]


                                November 5, 2002



Fair, Isaac and Company, Incorporated
200 Smith Ranch Road
San Rafael, CA  94903-1996

        Re:    Registration Statement on Form S-3

Ladies and Gentlemen:

We are acting as counsel for Fair, Isaac and Company, Incorporated, a Delaware
corporation ("Fair, Isaac"), in connection with Fair, Isaac's Registration
Statement on Form S-3 (the "Registration Statement").

By means of the Registration Statement, Fair, Isaac is registering the resale by
certain selling securityholders specified in the Registration Statement and the
prospectus associated therewith, from time to time, of (i) an aggregate of
1,234,078 shares of Fair, Isaac's common stock, par value $.01 per share (the
"Shares") issuable upon conversion of $68,479,000 aggregate principal amount of
5.25% Convertible Subordinated Notes due September 1, 2008 (the "Notes"),
pursuant to the Indenture dated as dated as of August 21, 2001 (the "Indenture")
between HNC Software Inc. ("HNC") and State Street Bank and Trust Company of
California, N.A. (the "Trustee"), as supplemented and amended by the First
Supplemental Indenture thereto dated as of August 5, 2002, among Fair, Isaac,
HNC and the Trustee (the "First Supplemental Indenture") and the Second
Supplemental Indenture thereto dated as of October 31, 2002 between Fair, Isaac
and the Trustee (the "Second Supplemental Indenture"), and (ii) the Notes.

In our capacity as such counsel, we have reviewed the Indenture, the First
Supplemental Indenture, the Second Supplemental Indenture, the form of the
Notes, the Registration Statement and such other corporate records, agreements,
documents and other instruments of Fair, Isaac, and satisfied ourselves as to
such other matters, as we have deemed necessary or appropriate as a basis for
this opinion. In such review, we have assumed the genuineness of all signatures,
the legal capacity of natural persons, the authenticity of all documents
submitted to us as originals, the conformity to the originals





Fair, Isaac and Company, Incorporated
November 5, 2002
Page 2



of all documents submitted to us as certified, conformed or photostatic copies
and the authenticity of the originals of such latter documents. We have assumed
that Fair, Isaac is validly existing and in good standing under the laws of
Delaware.

Based on the foregoing, and subject to the qualifications, assumptions and
limitations stated herein, and subject to bankruptcy, insolvency,
reorganization, moratorium or other similar laws affecting or relating to the
rights of creditors generally, and to general principals of equity, we are of
the opinion that (a) the Shares, when issued and sold in accordance with the
Indenture, the First Supplemental Indenture and the Second Supplemental
Indenture, will be legally issued, fully paid and non-assessable, and (b) upon
the due execution of the Second Supplemental Indenture and the consummation of
the Upstream Merger, as defined in the Second Supplemental Indenture, the Notes
will constitute the valid and binding obligations of Fair, Isaac.

We are members of the Bar of the States of California and New York and, for
purposes of this opinion, do not hold ourselves out as experts on the law of any
jurisdiction other than the law of the States of California and New York and the
Delaware General Corporation Law. This opinion is limited to matters governed by
the law of the States of California and New York and the Delaware General
Corporation Law (including the statutory provisions of the Delaware General
Corporation Law and also all applicable provisions of the Delaware Constitution
and reported decisions interpreting the Delaware General Corporation Law and the
Delaware Constitution).

We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement and to the reference to this firm under the heading
"Legal Matters" in the prospectus that is a part of the Registration Statement.
In giving this consent, we do not thereby admit that we are within the category
of persons whose consent is required under Section 7 of the Act or the rules and
regulations of the Commission thereunder.


                                       Very truly yours,

                                       /s/ Pillsbury Winthrop LLP